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QP38.L51  Physiology:  the  vita 


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PHYSIOLOGY : 
THE  VITAL  PROCESSES  IN  HEALTH 


BY 

FREDERIC  S.  LEE,  Ph.  D. 

ADJUNCT   PEOFESSOB   OF   PHYSIOLOGY,    COLUMBIA   COLLEGE,   NEW   YORK 


REPRINTED  FROM 
IN  SICKNESS  AND  IN  HEALTH 


COPYKIGHT,    1896,   BY   D.   APPLETON    &  Co. 


II. 

PHYSIOLOGY:   THE  VITAL   PROCESSES   IN   HEALTH. 
By   FREDERIC   S.   LEE,   Ph.  D. 

INTRODUCTION. 

The  physiology  of  an  organism  treats  of  the  healthy  working  of  the 
organism.  It  deals  with  the  living,  acting  body,  with  what  the  body 
does  and  how  it  does  it.  Anatomy  can  be  studied  best  upon  the  dead 
body.  Physiology,  however,  must  be  studied  chiefly  upon  the  living 
body,  since  in  death  the  action  ceases.  The  present  sketch  is  devoted 
particularly  to  the  physiology  of  man,  but  we  must  bear  in  mind  that 
physiology  is  a  very  broad  science  and  has  to  do  with  all  living  things, 
animals  or  plants.  Only  a  limited  range  of  observation  and  experi- 
mentation is  possible  upon  the  living  human  being,  hence  the  work  of 
the  physiologist  consists  chiefly  of  work  upon  animals  other  than  the 
human  species.  Human  physiology  consists  largely  of  careful,  well- 
guarded  inferences  from  the  results  of  such  experimental  work. 

The  reasonableness  of  such  a  method  of  inference  is  apparent  when 

one  accepts  as  a  fact  what  is  no  longer  doubted  by  scientific  men — that 

the  human  species  is  both  anatomically  and  physiologic- 

„  „    ,  . .  ally  a  form  that  has  been  derived  from  other  and  lower 

oj  Avoluhon.  J 

species  of  animals.  Such  a  belief  gives  a  unity  and 
harmony,  not  otherwise  possible,  to  the  facts  of  biology.  Man  is  physi- 
ologically interesting  in  himself ;  he  is  physiologically  more  interesting 
when  regarded  as  the  latest  and  most  complex  product  of  a  long  ancestry 
reaching  back  through  mammals,  reptiles,  fishes,  and  a  long  line  of 
simpler  animals,  to  the  most  primitive  forms.  The  study  of  these  ani- 
mals is  a  study  of  the  steps  by  which  man  has  arrived  at  his  present 
stage. 

Like  other  animal  bodies,  the  human  body  is  a  complicated  machine, 
adapted  for  doing  a  great  variety  of  work.  The  term  "  vital  energy  "  is 
often  heard — a  term  which  indicates  that  the  living  body  is  something 
fundamentally  different  from  other  machines  that  are  made  of  iron,  or 

85 


86        PHYSIOLOGY  :    THE   VITAL  PROCESSES  IN  HEALTH. 

steel,  or  brass.  During  the  present  century,  however,  it  has  been  proved 
that  all  kinds  of  energy  in  inorganic  nature — such  as  mechanical  work, 
heat,  light,  and  electricity — are  only  different  forms  of  a  universal  energy. 
Moreover,  it  has  been  shown  that  "  vital  energy "  is  not  distinct  from 
other  kinds,  and  that  the  actions  of  the  living  body — walking,  swimming, 
flying,  speaking,  the  circulation  of  the  blood,  and  probably  the  activities 
of  gland  cells,  of  brain  cells,  etc. — are  performed  according  to  the  same 
mechanical,  physical,  and  chemical  laws  that  apply  to  inorganic  matter. 
The  task  of  the  physiologist  consists,  then,  largely  in  a  study  of  the  me- 
chanics, physics,  and  chemistry  of  the  living  body.  The  goal  toward 
which  he  is  pressing  is  a  full  understanding  of  the  nature  of  life  itself. 
It  is  unnecessary  to  say  that  that  goal  is  still' far  distant. 

A  great  advance  that  has  taken  place  during  the  past  sixty  years  is 
the  proof  that  life  is  always  associated  with  a  certain  visible  material  sub- 
stance.    This  substance  is  called  protoplasm.     It  occurs 

ro  op  asm         •     muscles,  glands,  skin,  brain,  bone,  nerves,  and  all  the 

the  Physical  Basts  '  5.     .       '  '      .    .  '  '  '      ,  . 

of  Life  organs.     It  is  in  fact  the  living  substance  of  all  parts  of 

all  living  bodies.  It  is  always  associated  with  lifeless 
substance,  such  as  the  fluid  parts  of  a  body,  the  mineral  parts  of  bones,  the 
hard  substance  of  teeth,  the  nails,  the  hair,  and  the  microscopic  lifeless 
material  that  permeates  all  parts  of  every  body.  In  its  simplest  form 
protoplasm  is  a  colourless,  jelly-like,  nearly  transparent  substance,  some- 
what resembling  raw  white  of  egg.  It  is  a  mixture  of  several  chemical 
substances ;  it  has  a  variable  composition,  but  always  contains  carbon, 
hydrogen,  nitrogen,  oxygen,  and  sulphur.  It  differs  in  appearance  and 
composition  in  different  parts  of  the  body,  the  protoplasm  of  muscle  being 
identical  neither  with  that  of  gland  cells  nor  with  that  of  nervous  sub- 
stance. 

Dissection  shows  that  a  body  is  composed  of  definite  parts,  or  organs, 
each  of  which,  as  we  shall  see,  has  a  definite  function  to  perform.     Thus 

the  heart,  the  stomach,  the  eye,  the  liver,  and  the  brain 
dG  11  are  organs-     Each  organ  has  its  own  peculiar  structure, 

but  each  is  made  up  of  comparatively  few  structural 
substances.  These  substances  are  called  tissues,  and  each  tissue — such  as 
connective  tissue,  muscle  tissue,  fat  tissue — has  its  own  work  to  do.  Ex- 
amination of  the  tissues  with  the  microscope  shows  that  each  is  composed 
partly  of  lifeless  substance,  usually  in  small  quantity,  and  chiefly  of  minute 
living  particles  of  protoplasm,  the  cells  (Fig.  1).  Cells  vary  greatly  in 
shape  and  size  and  in  the  work  that  they  do,  but  in  any  one  tissue  they 
are  similar  in  structure  and  function.  The  work  of  a  body  is  the  sum  of 
the  work  of  its  individual  cells,  and  the  fundamental  problems  in  physi- 
ology lead  back  to  protoplasm  and  the  cells. 

Every  human  body  consists  at  first  of  a  single  cell  within  the  body  of 


DIVISION   AND   DIFFERENTIATION   OF  CELLS. 


87 


the  mother.  In  growth  the  single  cell  divides  into  two  cells,  the  two  into 
four,  the  four  into  eight,  and  the  process  continues  until  in  the  adult 
millions  of  cells  exist.  Along  with  the  increase  in  number  there  occurs  a 
differentiation  in  form  and  a  division  of  labour  among  the  various  cells, 
such  that  some  come  to  perform  the  various  muscular  movements,  others 
prepare  digestive  substances,  others  remove  waste  matter  from  the  blood, 
others  control  the  breathing,  still  others  do  the  thinking,  some  are  affected 


Fig.  1. — Typical  cells  from  the  human  body. 
Each  cell  consists  of  granular  or  striated  protoplasm,  and  contains  a  denser  protoplasmic  mass,  the 
nucleus.  The  nucleus  is  round  or  oval,  and  appears  darker  or  lighter  than  the  rest  of  the  cell. 
A,  seven  pigment  cells  from  retina  (Schultze) :  B,  four  epithelium  cells  from  intestine— one 
swollen  with  mucus  represents  a  "goblet  cell"  (Frey);  C,  unstriped  muscle  cell  (Arnold); 
D,  nerve  cell  from  brain  (Gage) ;  E,  ciliated  epithelium  cell — the  cilia  constitute  the  brushliko 
upper  end  (Rosenthal);  F,  three  "prickle-cells"  from  skin  (Robinson);  G,  three  gland  cells 
from  liver— the  heavy  dark  lines  represent  bile  ducts  (Kolliker). 

by  waves  of  light,  some  by  waves  of  sound,  and  thus  we  perceive  that  in 
this  complicated  human  machine  each  part  has  its  own  work  to  do,  and 
the  division  of  labour  is  far-reaching. 

Protoplasm  is  said  to  be  irritable  or  excitable — that  is,  it  is  capable  of 
being  thrown  into  activity  by  proper  stimuli.  Every  kind  of  cell,  or 
tissue,  or  organ  has  its  own  natural  method  of  stimulation  and  its  own 
natural  response  or  function.  Every  vital  action  is  a  response  to  some 
kind  of  stimulus. 


S3  PHYSIOLOGY  :    THE   VITAL  PROCESSES  IN  HEALTH. 

The  leading  functions  of  the  body,  together  with  their  chief  agents, 
may  be  grouped  as  follows : 

Functions.  Organs  and  other  Agents. 

f  Alimentation   \  Alimentary  canal:  consisting  of  mouth,  oesophagus,  stomach, 
j      small  intestine  and  large  intestine  ;  liver ;  pancreas. 
(  Blood  system :  consisting  of  heart,  arteries,  capillaries  and 
v  t  >■       J  Circulation..  \      veins;  lymphatic  system:  consisting  of  lymphatic  vessels 
Nutrition.  <  [      and  lvmphatic  giands  .  blood,  lymph. 

Respiration . .  Trachea,  lungs. 

Metabolism..  All  living  cells. 

Excretion.. . .  Lungs,  kidneys  and  accessory  organs,  skin. 

Motion All  muscles. 

Co-ordination Brain,  spinal  cord,  nerves,  sympathetic  nervous  system. 

Activity  of  special  senses.,  i  ^e>  fl  01'Sans,  of  sme11;.  o£  taste'  of  touch>  of  temp«»ture, 
"       J         r  a  j      and  of  muscular  sensations. 

Support Bones,  cartilage,  connective  tissue. 

p         ,  (  Female:  ovaries  and  accessory  organs. 

Keproauction -J  Male.  testeg  and  accessorv  organs. 


CHAPTER   I. 

NUTRITION. 

Like  any  other  machine  in  action,  the  living  human  body  constantly 
gives  off  energy  to  the  outside  world  in  the  form  of  heat  and  mechanical 
work,  and,  moreover,  its  own  material  is  being  constantly  used  up.  It 
requires  food  to  replace  these  two  losses  of  energy  and  substance.  The 
story  of  nutrition  is  a  long  one.  It  tells  how  the  body  prepares  for  its 
use  the  food  that  is  given  to  it  (digestion) ;  how  the  prepared  food  is 
taken  into  the  distributing  apparatus  (absorption) ;  how  it  is  carried  to 
the  living  cells  (circulation) ;  how  it  is  used  there  for  the  production  of 
new  protoplasm,  and  how  the  protoplasm  in  action  is  destroyed,  leaving 
waste  matters  (metabolism) ;  and,  finally,  how  the  wastes,  being  harmful 
to  the  body,  are  removed  from  it  (excretion).  These  subjects  will  now 
be  considered  in  detail. 

Section   I. 

ALIMENTATION. 

Alimentation  consists  of  the  two  processes  of  digestion  and  absorp- 
tion.    Digestion  comprises  the  chemical  and  physical  changes  which  the 

food  undergoes  by  way  of  preparation  for  entrance  into 
Alimentation  in       ,,  ,    »'  ij.it-  v  i  a  u 

c         ,  the  tissues,  and  tor  use  by  the  living  substance.     Ab- 

sorption is  the  process  of  the  passage  of  digested  food 
into  the  blood  and  lymph,  whence  it  is  carried  to  the  tissues.  Alimen- 
tation takes  place  in  the  alimentary  canal  (Fig.  2),  which  is  a  long  tube 


THE  HUMAN   ALIMENTARY   CANAL. 


89 


extending  through  the  body,  opening  above  at  the  mouth  and  below  at 

the  anus  ;  its  walls  consist  of  muscle  with  glands,  blood-vessels,  and  lymph- 
vessels.     The  successive  parts  of 

the  canal  are  known  as  mouth, 

pharynx,    oesophagus    or    gullet, 

stomach,  small  intestine  and  large 

intestine.     To  it  are  joined,  by 

ducts,    several     large     glands  — 

namely,  the  three  pairs  of  sali- 
vary  glands,   the   pancreas,   and 

the  liver ;   all  of  these  in  their 

origin  are  outgrowths  from  the 

canal,  but  in  the  adult  body  they 

lie    outside    of    its    walls.      The 

food  enters   at   the   mouth,  and 

is  propelled  along  the  canal  by 

the  contraction  of  the  muscular 

walls ;    the   glands   secrete — that 

is,    manufacture   and    pour  into 

the  canal  various  fluids  that  differ 

in  nature  in  various  parts  of  the 

canal,  and  whose  office  it  is  to 

digest  the  food  ;  the  blood-vessels 

and    the    lymph-vessels,    besides 

bringing  blood  and  lymph  to  the 

walls   of   the  canal,  receive   the 

digested  food  and  bear  it  away 

to   all    parts   of  the  body ;    the 

indigestible,  the  undigested,  and 

the  innutritions  substances  in  the 

canal  undergo  chemical  changes, 

and  finally  are  expelled  from  the 

body  as  excrement. 

The   common   foods,  though 

apparently  so  different  in  char- 
acter from  each 
Food  Stuffs.  ,  , 

other,  are  found 

by  chemical  analysis  to  be  mix- 
tures of  a  small  number  of  sub- 
stances known  as  food  stuffs. 
The  most  common  food  stuffs  are  proteids  (the  most  important  con- 
stituent of  all  meats,  fish  and  eggs)  ;  albuminoids  (such  as  gelatin  ;  they 
are  closely  related  to  proteids,  and  hereafter  will  be  included  with  them) ; 


Fig.  2. — Human  alimentary  canal. 
oesophagus;  b,  stomach:  c,  cardiac  orifice;  f/, 
pylorus  ;  e.  small  intestine  ;  /',  biliary  duct  and 
gall  bladder;  g,  pancreatic  duct;  A,  i,  j,  i, 
large  intestine;  h,  ascending  colon;  i,  trans- 
verse colon;  j\  descending  colon;  k,  rectum. 
At  junction  of  e  and  h  is  ileo-caaeal  valve,  be- 
low which  is  vermiform  appendix.     (Dalton.) 


90         PHYSIOLOGY:    THE  VITAL  PROCESSES  IN  HEALTH. 

carbohydrates  (such  as  starch  and  sugar,  abundant  in  bread,  pastry,  and 
vegetables) ;  fats  (abundant  in  meats,  butter,  and  milk) ;  water  and  salts 
(such  as  table  salt  and  numerous  other  salts).  For  a  discussion  of  the 
characters  and  value  of  these  various  food  stuffs  the  reader  is  referred  to 
the  article  in  this  volume  on  Hygiene. 

Water  and   the  salts  that   are  dissolved   in  the  food   are  absorbed 
unchanged  into  the  blood.     Of  the  other  food  stuffs,  proteids  and  carbo- 
hydrates are  chemically  changed  by  the  digestive  pro- 

iges  ,omn        ce      the  former  into  closely  related  bodies  called  pro- 
Cteneral.  .        . 

teoses,  peptones,  and  derivatives  of  the  peptones,  the 

carbohydrates  mainly  into  a  form  of  sugar  called  maltose,  together  with 

a  small  quantity  of  a  starch  called  dextrin.     By  reason  of  the  unsettled 

questions  regarding  the  true  nature  of  some  of  these  digested  substances, 

it  will  suffice  hereafter  to  speak  of  all  the  products  of  proteid  digestion 

simply  as  peptone,  and  of  all  the  products  of  carbohydrate  digestion  as 

sugar.     Contrary  to  what  is  found  in  most  of  the  food  stuffs,  peptone 

and  sugar  are  soluble,  and  hence  when  dissolved  in  the  water  of  the  food 

are  fit  for  absorption.     The  digestion  of  the  fats  consists  in  part  in 

changing  them  chemically  into  fatty  acid,  glycerin,  and  soap,  and  in  part 

in  simply  breaking  them  up  mechanically  into  minute  droplets,  this  latter 

process  being  called  emulsification  ;  after  these  changes  the  fats  are  ready 

for  absorption. 

The  digestive  fluids  are  manufactured  in  the  glands  of  the  alimentary 
canal,  and  are  poured  out  into  it.  They  are  five  in  number — viz.,  saliva, 
gastric  juice,  intestinal  juice,  pancreatic  juice,  and  bile. 
Each  consists  chiefly  of  water  together  with  a  small 
quantity  of  dissolved  solids,  among  which,  except  in  the  bile,  is  one  or 
more  of  a  peculiar  class  of  bodies  called  enzymes  or  unorganised  fer- 
ments, to  which  the  digestive  property  of  the  fluids  is  due.  The  enzymes 
are  peculiar  chemical  compounds,  the  exact  constitution  of  which  is  un- 
known, but,  when  mixed  with  the  food  stuffs,  they  produce  extensive 
chemical  changes  in  the  latter  without  being  in  themselves  greatly  altered. 

Saliva  is  produced  by  the  minute  glands  in  the  walls  of  the  mouth 
and  the  three  pairs  of  large  salivary  glands,  the  parotid,  the  submaxillary, 
and  the  sublingual  (see  The  Anatomy  of  the  Human  Body).  It  is  very 
watery,  containing  less  than  0'6  per  cent,  of  solids.  Among  the  latter 
are  mucin,  which  gives  to  the  saliva  its  slightly  slimy  quality,  and  assists 
in  the  swallowing  of  the  food ;  and  ptyalin,  an  enzyme  that  changes 
starch  into  sugar.  Saliva  is  produced  in  small  quantity  at  all  times,  but 
more  abundantly  when  food  is  in  the  mouth. 

Gastric  juice  is  produced  by  the  innumerable  small  glands  lying  in 
the  walls  of  the  stomach.  It  is  watery,  colonrless,  sour,  and  contains 
three   per  cent,   of   solid  matter.     Its  sourness  is  due  to  hydrochloric 


THE  DIGESTIVE  FLUIDS  AND  GLANDS.  91 

(muriatic)  acid.  It  contains  two  enzymes — viz.,  pepsin,  which  changes 
proteid  to  peptone,  and  rennin,  which  curdles  milk.  The  glands  of  the 
stomach  do  not  act  continuously,  but  are  stimulated  to  activity  by  the 
food  that  has  been  swallowed. 

Intestinal  juice  is  secreted  by  the  glands  in  the  walls  of  the  small 
intestine.  It  is  not  abundant,  is  a  yellowish  alkaline  fluid,  and  contains 
at  least  two  kinds  of  enzymes,  one  like  the  ptyalin  of  saliva,  capable  of 
converting  starch  into  sugar,  and  the  other  that  changes  maltose  and 
other  complex  sugars  into  dextrose,  a  sugar  of  simpler  composition. 

Pancreatic  juice  is  manufactured  by  the  pancreas ;  it  resembles  saliva 
in  appearance,  but  contains  some  thirteen  per  cent,  of  solids,  among  which 
are  sodium  carbonate  and  three  powerful  enzymes.  Of  these,  trypsin, 
like  pepsin,  converts  proteids  into  peptones,  and,  moreover,  splits  up  some 
of  the  peptones  into  other  bodies,  the  reason  for  which  is  not  quite  clear ; 
amylopsin,  like  ptyalin,  converts  starch  into  sugar ;  steapsin  splits  up 
fat  into  fatty  acid  and  glycerin.  The  fatty  acid  thus  produced,  with  the 
sodium  carbonate  present,  forms  soaps  that  are  capable  of  emulsifying 
fats.     Hence  pancreatic  juice  may  digest  all  kinds  of  food  stuffs. 

Bile  is  produced  by  the  liver.  It  is  a  yellowish  or  greenish-yellow, 
somewhat  slimy,  alkaline  fluid,  and  contains  two  to  three  per  cent,  of 
solids  that  are  of  great  variety,  but  apparently  do  not  include  any  enzyme. 
Its  secretion  goes  on  continually,  six  hundred  to  eight  hundred  and  fifty 
cubic  centimetres  (averaging  a  pint  and  three  quarters)  being  produced  in 
twenty-four  hours.  It  is  stored  up  in  the  gall  bladder  for  a  time,  and 
passed  out  into  the  intestine  during  the  course  of  digestion.  The  con- 
stituents of  bile  are  largely  waste  products,  thrown  off  by  the  living  sub- 
stance throughout  the  body,  removed  from  the  blood  by  the  liver  cells, 
and  cast  out  into  the  intestine  for  expulsion.  But  in  addition  to  its  value 
as  an  excretion,  bile  is  an  important  agent  in  alimentation,  aiding  greatly, 
in  a  manner  not  yet  fully  explained,  the  digestion  and  absorption  of  the 
fats.      . 

The  glands  that  manufacture  the  digestive  fluids  consist  of  chemically 

active  secreting  cells  that  are  richly  supplied  with  blood   and   lymph. 

They  have  the  power  of  removing  from  these  fluids 
Digestive  Glands.  ,        .  ,.       ,.        ,.        „    .  -,      ,., 

some  components  ot  the  digestive  fluids,  like  water  and 

salts,  and  other  substances  from  which  they  manufacture  the  remaining 

components.     The  resulting  secretion  passes  on  into  the  alimentary  canal. 

The  intestinal  glands,  the  simplest  of  all,  consist  each  of  a  little  pocket 

opening  into  the  intestine  and  with  walls  of  secreting  cells  (Fig.  3).     In 

the   more  complex   pancreas,  liver,  and   salivary  glands   the   secreting 

pockets,  or  alveoli,  are  greatly  branched,  tortuous  channels  uniting  into 

one  tube,  the  duct,  which  conveys  the  secretion  to  the  alimentary  canal. 

The  glands  are  among  the  most  active  of  all  the  living  parts  of  the  body, 


92 


PHYSIOLOGY:    THE   VITAL  PROCESSES  IN  HEALTH. 


and  are  carefully  controlled  and  regulated  by  the  nervous  system.     The 
digestive  ferments  may  readily  be  extracted  from  glands  that  have  been 
removed  from  the  bodies  of  dead  animals,  and  may  be  used  for  the  manu- 
facture of  artificial  digestive  fluids.     With  such 
fluids  the  processes  of  digestion  of  foods  may  be 
carried   on  under  observation   in  vessels   in   the 
laboratory.     This  has  been  one  of  the  most  fruit- 
ful methods  of  studying  the  subject.     Gland  ex- 
tracts form  the  basis  of  many  substances  used  by 
the  physician  for  the  relief  and  cure  of  dyspepsia. 
Let  us  now  trace  in  order  the  events  of  diges- 
tion.    Food  is  put  into  the  mouth,  where  its  pres- 
ence excites  the  salivary  glands  to  manufacture 

and  pour  out  upon  it  the  saliva. 
Digestion  in  the       The    food    ■       Qr  u    tQ    b 

Mouth  and  D  „• '     . 

Stomach.  chewed  thoroughly,  so  as  finally 

to  divide  it  and  thus  to  allow 
the  digestive  fluids  to  permeate  it  readily.  The 
saliva  mixes  with  it,  and  the  chemical  changes  of 
digestion  begin  by  the  conversion  of  some  of  the 
carbohydrates  present  into  sugar.  By  muscular 
action  the  food  is  swallowed — that  is,  is  squeezed 
along  through  the  pharynx  and  the  gullet  into 
the  stomach,  where  it  remains  for  a  time  varying 
from  a  few  minutes  to  a  few  hours.  "Waves  of 
contraction  pass  over  the  muscular  walls ;  the 
food,  being  squeezed  here  and  there,  is  kept  in 
constant  slow  motion.  The  gastric  glands  are 
stimulated  to  activity,  and  the  gastric  juice  oozing 
forth  mixes  with  the  mass  of  food  and  converts 
it  into  a  half-fluid  mass  called  chyme.  The  acid  of  the  gastric  juice  puts 
a  stop  to  further  action  of  the  swallowed  saliva.  Carbohydrates  are  hence 
here  unchanged,  but  the  proteids  are  in  great  part  altered  to  peptones. 
At  intervals  the  passageway  into  the  small  intestine — the  pylorus — opens 
and  allows  the  chyme  to  flow  out  into  the  duodenum.  Thus  gradually 
the  stomach  completes  its  work,  and  transfers  its  contents  to  the  succeed- 
ing part  of  the  alimentary  canal. 

In  the  small  intestine  the  processes  of  muscular  action,  glandular 
secretion,  and  digestion  are  continued.     The  food  is  moved  about  and 

passed  slowly  along.     The  gastric  juice  ceases  to  act, 
Diqestion  in  the       .,..,..  ,  ,.       .    .   '  jjjj.ii 

c     „  T  ,  i-  intestinal  liuce  and  pancreatic  luice  are  added  to  the 

Small  Intestine.  J  r  J  -  - 

mass,  and  the  liver  adds  its  contribution  of  bile.     Ihe 
carbohydrates  that  were  unaffected  by  the  saliva  are  changed  into  sugar. 


Fig.  3. — Gland  from  the 
human  intestine. 

The  gland  is  dilated  at  its 
closed  end  ;  at  the  other 
end  it  opens  into  the  in- 
testine. Each  of  the  se- 
creting cells  of  which  the 
wall  consists  contains  a 
deeply  shaded  nucleus. 
The  clear  spaces  repre- 
sent cells  laden  with  mu- 
cus.    (Flemming.) 


PROCESSES  OF  DIGESTION.  93 

The  proteids  that  were  unaltered  by  the  gastric  juice  become  peptones, 
and  are  even  in  part  decomposed  into  simpler  substances.  The  fats  are  for 
the  first  time  affected,  being  in  small  part  decomposed  into  fatty  acid  and 
glycerin,  in  greater  part  divided  mechanically  into  fine  droplets.  This 
em  unification  is  assisted  by  the  soaps  that  are  formed  by  a  union  of  the 
fatty  acids  and  the  alkalies  present  in  the  digestive  fluids.  Thus  all 
classes  of  food  stuffs  are  attacked  in  the  small  intestine ;  no  other  part  of 
the  whole  canal  forms  so  active  a  digestive  laboratory  as  this  part.  Be- 
sides the  changes  already  spoken  of,  due  chiefly  to  the  enzymes,  the  ob- 
ject of  which  is  to  make  ready  the  nutriment  for  entrance  into  and  use 
by  the  protoplasm,  fermentative  changes  of  the  digested  products  take 
place  of  a  destructive  character,  and  due  apparently  to  the  agency  of 
living  microbes  that  are  always  present  in  the  intestine.  The  significance 
of  this  apparent  destruction  of  nutritive  substance  is  not  understood.  The 
bacteria  are  not  disease-producing  germs;  on  the  contrary,  they  may  pos- 
sibly be  of  considerable  value  to  the  body,  but  the  role  that  they  play 
needs  investigation.  Absorption  of  the  digested  food  into  the  blood  and 
lymph  is  most  active  in  the  walls  of  the  small  intestine,  and  by  the  time 
the  semifluid  mass  has  reached  the  ileo-csecal  valve  it  is  robbed  largely  of 
its  nutritious  components. 

In  the  large  intestine  no  new  physiological  features  are  added.     Feeble 
digestion,  fairly  active  absorption  of  digestive  products  and  of  water, 
and   marked   fermentation   by  bacteria,  are  the  main 
r        j  t   ,-  events ;   and  the  undigested  and  the  indigestible  sub- 

stances are  passed  on  into  the  rectum  for  expulsion  from 
the  body.  The  large  intestine  appears  to  be  physiologically  more  impor- 
tant in  herbivorous  animals,  like  cows  and  sheep,  where,  in  accordance 
with  the  enormous  quantity  of  food,  it  is  relatively  large.  It  was  doubt- 
less larger  and  more  important  in  the  ancestors  of  the  human  race.  But 
in  man  it  appears  to  be  undergoing  degeneration,  as  is  shown  especially  in 
the  part  known  as  the  vermiform  appendix.  This  appendage,  probably 
valuable  in  the  digestive  work  of  the  ancestors  of  man, 

'        \.  seems  to  be  devoid  of  marked  function  in  man  at  the 

Appendix. 

present  time,  and  is  especially  prone  to  inflammatory 
processes  set  up  by  the  presence  within  it  of  irritating  substances  brought 
there  by  the  food.  It  would  seem  a  blessing  if  evolution  could  hasten 
the  disappearance  of  this  apparently  useless  structure. 

A  word  now  as  to  the  absorption  of  the  digested  food  stuffs,  and  the 
outline  of  the  story  of  alimentation  will  be  completed.     Within  the  ali- 
mentary canal  the  food  is  no  more  a  part  of  the  body 
than  if  it  were  upon  the  outer  surface.     As  has  been 
insisted,  digestion  within  the  alimentary  canal  is  merely  a  preparation  of 
the  food  for  entrance  into  the  actual  living  substance.     The  cells  that 


91         PHYSIOLOGY  :    THE   VITAL  PROCESSES  IN   HEALTH. 

line  the  canal  are  nourished  by  absorbing  food  directly  from  the  mass 
that  lies  in  contact  with  them.  More  distant  cells  require  nutriment  to 
be  brought  to  them,  and  this  is  one  important  office  of  the  blood  and 
lymph  with  which  the  walls  of  the  alimentary  canal  are  so  saturated. 
Thin-walled  blood  capillaries  and  lymph  capillaries  abound  there,  and 
during  and  after  digestion  the  dissolved  food  stuffs  make  their  way  into 
them.  Absorption  takes  place  very  slightly,  if  at  all,  in  the  stomach ; 
it  is  at  its  greatest  height  in  the  small  intestine,  and  it  is  active  even 
throughout  the  large  intestine.  The  method  of  absorption  is  not  wholly 
clear.  Probably  physical  processes  play  a  leading  part ;  but  it  is  a  ques- 
tion whether  the  living  cells  that  line  the  intestine,  and  through  which 
the  food  must  pass  on  its  way  to  the  capillaries,  may  not  in  some  manner 
engage  actively  in  the  process.  Sugar  and  fats  pass  through  them  un- 
changed, peptone  is  mysteriously  altered  chemically  in  its  passage.  This 
altered  peptone,  the  sugar,  and  the  greater  part  of  the  salts  and  the  water 
go  directly  into  the  blood.  Fat  goes  into  the  lymph  and,  by  way  of  the 
lymphatic  vessels  and  the  thoracic  duct,  finally  into,  the  blood  system. 
Thus  all  nutriment  that  is  not  used  by  the  living  substance  in  the  wall 
of  the  alimentary  canal  finds  its  way  sooner  or  later  to  the  blood,  the 
great  carrier  and  distributer  of  matter  and  energy.  Our  next  section 
deals  with  this  circulating  mechanism. 


Section   II. 

CIRCULATION. 

In  an  organism  that  consists  of  one  cell  or  a  few  cells,  food,  when 

once  digested,  permeates  all  parts  readily.     In  larger  organisms  this  is 

impossible,  and  hence  special  mechanisms  must  exist  for  the  transfer  of 

nutriment  from  the  organs  of  digestion  to  the  more  distant  parts.     In 

the  growth  of  all  except  the  simplest  animals  such  a  mechanism,  some 

sort  of  a  distributing  system,  is  developed.     It  is  simple 
Circulation  in  -,  ,     .      .,     ,  ,  .      ,,      , 

„      ,  ,  and  crude  in  its  beginnings,  and  in  the  lower  organisms 

remains  always  simple  and  crude.  It  becomes  more 
complex  and  perfected  as  the  animal's  size  increases  and  his  structure 
becomes  more  complex,  until  in  the  completed  circulatory  system  of  the 
highest  animals  and  man  we  have  an  apparatus  that  is  wonderfully 
adapted  to  perform  its  needed  work,  and  responsive  in  a  high  degree  to 
the  demands  of  the  other  physiological  systems.  The  transfer  of  nutri- 
ment is  not  the  only  important  function  of  the  organs  of  circulation.  Of 
equal  value  are  the  transfer  of  oxygen  without  which  the  living  substance 
can  not  act,  and  the  removal  of  waste  products  from  the  tissues  to  the 
organs  of  excretion.     Furthermore,  it  must  be  borne  in  mind  that  the 


CIRCULATION   IN   MAN   AND  IN  ANIMALS. 


Blood  and  its 
Constituents. 


among  other 


food  contains  not  only  material  for  the  manufacture  of  new  protoplasm, 
but  also  in  a  latent  form  all  the  energy  of  which  the  body  makes  use ; 
hence  the  circulation  is  the  medium  for  the  transference  of  energy  from 
one  part  of  the  body  to  another.  Finally,  as  we  shall  discuss  later,  the 
6ame  system  acts  to  keep  the  temperature  of  the  various  portions  of  the 
body  uniform.  A  review  of  the  circulation  comprises  a  study  of  the  two 
fluids  which  serve  as  carriers  of  the  oxygen,  the  food,  the  energy,  and 
the  wastes — viz.,  the  blood  and  the  lymph,  and  a  study  of  the  organic 
mechanisms  by  which  they  are  made  to  move. 

Blood  may  be  regarded  as  a  tissue  consisting  of  a  lifeless,  slightly  yel- 
lowish liquid,  the  plasma,  and  living  cells,  the  corpuscles.  The  plasma 
contains  in  solution  the  food  and  the  waste  matters,  hence 
its  composition  is  complex.  Besides  water,  it  contains 
eight  to  nine  per  cent,  of  solid  substances,  comprising 
bodies  proteids,  sugar,  fats,  and  salts.  Some  of  these  con- 
stituents originate  in  the  di- 
gested food  stuffs.  It  con- 
tains also  a  gas,  carbonic  acid. 
The  corpuscles  are  of 
three  kinds,  the  red,  the 
white  or  colourless,  and  the 
blood-plates,  which  are  also 
colourless  (Fig.  4).  The  red 
corpuscles  are  most  numer- 
ous, it  being  estimated  that 
in  one  cubic  millimetre  of 
^ood  {yjtts  °f  a  cu°ic  inch) 
there  exist  five  millions  in 
man  and  about  a  half  million 
less  in  woman.  The  number 
in  the  whole  body  is  incon- 
ceivably great.  They  are  bi- 
concave round  disks  about 
¥2inr  °f  an  *nCQ  *n  diameter, 
and  consist  of  a  bit  of  pro- 
toplasm coloured  by  a  red- 
dish pigment  called  haemo- 
globin. Haemoglobin  gives 
to  the  blood  its  colour,  and 
is  one  of  its  most  important 

constituents  by  reason  of  having  a  special  attraction  for  oxygen, 
the  blood  in  its  course  passes  through  the  lungs,  its  haemoglobin  absorbs 
oxygen  from  the  air  that  has  been  breathed  in,  and  holds  it  until  the  gas 


Fig.  4.- 


Ked  and  white  corpuscles  of  the  blood, 
magnified. 
.•(,  moderately  magnified :  the  red  corpuscles  are  seen  lying 
in  rouleaux ;  at  a  and  a  are  seen  two  white  corpuscles ; 
J3,  red  corpuscles  much  more  highly  magnified,  seen 
in  face  ;  G,  ditto,  seen  in  profile ;  D,  ditto,  in  rouleaux, 
rather  more  highly  magnified;  J?,  a  red  corpuscle 
swollen  into  a  sphere  by  imbibition  of  water;  F,  a 
white  corpuscle  magnified  same  as  B\  67,  ditto,  throw- 
ing out  some  blunt  processes;  A',  ditto,  treated  with 
acetic  acid,  and  showing  nucleus,  magnified  same  as p ; 
7/,  red  corpuscles  puckered  or  crenate  all  over  ;  7,  ditto 
at  the  edge  only.    (Huxley.) 


When 


96         PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN  HEALTH. 

is  required  by  the  tissues.  The  task  of  the  red  corpuscles  is  hence  that 
of  carriers  of  oxygen.  In  cases  of  murder  or  other  crimes,  where  a  stain 
is  suspected  to  be  caused  by  blood,  an  examination  with  the  microscope, 
by  revealing  the  red  corpuscles,  will  decide  the  question.  Even  if  the 
corpuscles  have  been  destroyed,  the  presence  of  haemoglobin  can  easily 
be  detected  by  proper  chemical  methods.  Human  red  blood-corpuscles 
may  be  distinguished  readily  from  those  of  fishes,  frogs,  reptiles,  or 
birds,  but  not  conclusively  from  those  of  higher  animals  except  the 
camel.  Human  hasmoglobin  is  not  distinguishable  from  that  of  other 
animals. 

The  white  corpuscles  are  colourless  cells  of  somewhat  irregular  shape, 
roughly  spherical,  about  a4100  of  an  inch  in  diameter.  They  are  much 
fewer  in  number  than  the  red  corpuscles.  They  have  the  peculiar  power 
of  creeping  about  through  the  walls  of  the  blood  capillaries,  and  in  among 
the  cells  of  the  tissues  throughout  the  body.  Their  specific  work  is  not 
fully  known.  It  has  been  thought  that  they  may  be  of  special  benefit 
to  the  body,  when  attacked  by  germ  diseases,  by  absorbing  the  bacterial 
germs  into  their  substance  and  destroying  them. 

The  blood-plates  were  discovered  only  recently.  They  are  minute, 
colourless,  spherical,  or  elliptical  bodies,  that  go  to  pieces  very  easily. 
This  happens  especially  when  blood  is  shed,  in  which  case  they  seem 
to  aid  in  the  clotting  of  the  blood.  Their  office  within  the  body  is 
unknown. 

The  quantity  of  blood  in  a  healthy  man  of  one  hundred  and  fifty 
ponnds  is  about  five  and  a  half  quarts. 

"When  blood  is  shed  it  has  the  peculiar  property  of  clotting  or  thicken- 
ing into  a  jelly-like  mass.  The  special  value  of  this  property  lies  in  its 
use  in  stopping  the  loss  of  blood  from  wounds ;  if  it  were  not  for  this, 
the  slightest  injury  to  the  skin  might  result  in  bleeding  to  death.  Clot- 
ting consists  in  the  formation  irom  fibrinogen,  one  of  the  proteids  exist- 
ing in  the  plasma,  of  an  insoluble  substance  called  fibrin.  Fibrin  exists 
in  the  clot  in  the  form  of  fine  whitish  threads  that  extend  in  all  directions 
and  form  a  spongy  network.  This  holds  in  its  meshes  the  corpuscles, 
and  the  whole  forms  an  effectual  plug  for  the  wound.  By  the  spontane- 
ous shrinking  of  the  fibrin  threads  a  yellowish  fluid,  that  is  really  plasma 
minus  the  fibrin  and  is  called  serum,  is  squeezed  out. 

Lymph  is  a  colourless  fluid,  occurring  partly  in  the  lymphatic  vessels 
and  partly  in  the  spaces  between  the  cells  of  the  tissues.  It  thus  comes 
more  closely  into  contact  with  the  living  protoplasm 
than  the  blood,  the  latter  never  leaving  its  vessels.  It 
is  much  like  blood  in  composition,  but  lacks  the  red  corpuscles  and  the 
blood-plates.  After  a  meal  of  fat  the  lymph  in  the  lymphatics  of  the 
intestine  is  loaded  with  fat  droplets,  and  is  pure  white  in  colour  like  milk, 


CIRCULATORY  ORGANS   OF  THE  BLOOD. 


97 


Circulation 
of  the  Blood. 


Th.V. 


mr. 


the  whiteness  of  which  is  due  to  the  contained  droplets  of  hutter.  Such 
lymph  is  called  chyle,  and  the  lymphatics  in  that  region  are  known  as 
laoteals. 

The    blood    is    moved    through    the 
blood-vessels  by  the  contractions  of  the 

heart.     The  motion  was 

thought  formerly  to  be 

a  sluggish  oozing  from 
the  heart  to  the  tissues.  But  in  1628  it 
was  shown  by  the  Englishman,  William 
Harvey,  physician  to  Charles  I.,  that 
every  particle  of  blood  makes  a  complete 
circuit  of  the  blood-vessels,  and  returns 
ultimately  to  its  place  of  starting ;  that 
the  blood  moves,  so  to  speak,  in  a  circle, 
and  since  Harvey's  time  the  movement 
has  been  spoken  of  as  the  circulation  of 
the  blood.  The  circulatory  organs  of  the 
blood  system  consist  of  the  heart,  the 
arteries,  the  capillaries,  and  the  veins. 
The  heart  pumps  the  blood  into  the  main 
arteries ;  along  these  the  liquid  courses 
toward  the  tissues,  passing  into  smaller 
and  smaller  arteries,  and  finally  into  the 
minute  capillaries ;  in  the  capillaries  it 
permeates  the  tissues  and  courses  among 
the  living  cells;  from  the  capillaries  it 
passes  into  the  small  veins ;  these  unite 
into  larger  and  larger  vessels,  and  finally, 
by  a  few  large  venous  trunks,  the  blood 
returns  to  the  heart  again.  For  the 
structure  and  arrangement  of  the  circu- 
latory organs,  and  for  the  general  plan  of 
the  circulation,  the  reader  is  referred  to 
the  article  on  The  Anatomy  of  the  Hu- 
man Body,  Sections  IV  and  V,  and  to 
the  accompanying  figure  (Fig.  5).  In 
order  to  make  the  complete  circuit  of 
the  circulatory  organs,  any  particle  of 
blood  must  traverse  two  sets  of  arteries, 
capillaries,  and  veins,  and  all  four  cham- 
bers of  the  heart.     The  path   from   the 


Fig.  5. — Diagram  of  the  heart  and 
vessels,  with  the  course  of  the  cir- 
culation, viewed  from  behind,  so 
that  the  proper  left  of  the  ob- 
server corresponds  with  the  left 
side  of  the  heart  in  the  diagram. 

L.A.,  left  auricle  ;  L.  V.,  left  ventricle  ; 
Ao.,  aorta ;  A1,  arteries  to  the  upper 
part  of  the  body ;  A2,  arteries  to  the 
lower  part  of  the  body  ;  H.A.,  hepatic 
artery,  which  supplies  the  liver  with 


part  of  its  blood  ;  V1,  veins  of  the  up- 
per part  of  the  body  ;  V2,  veins  of  the 
lower  part   of  the  body ;    V.P.,  vena 


left  ventricle  through  the  blood-vessels  of 


portte  ;  //  V.,  hepatic  vein ;  V.  C.I.,  in- 
ferior vena  cava;  V.C.S.,  superior  vena 
cava;  R.A.,  right  auricle;  B.  V.,  right 
ventricle  ;  PA.,  pulmonary  artery ; 
Lg.,  lung  ;  P.  V.,  pulmonary  vein  ;  Let., 
lacteals ;  Ly.,  lymphatics ;  Tk.  D., 
thoracic  duct ;  At.,  alimentary  canal; 
Lr.,  liver.  The  arrows  indicate  the 
course  of  the  blood,  lymph,  and  chyle. 
The  vessels  which  contain  arterial 
blood  have  dark  contours,  while  those 
which  carry  venous  blood  have  light 
contours.     (Huxley.) 


98 


PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN   HEALTH. 


the  body,  except  the  lungs  and  to  the  right  auricle,  is  called  the  "  greater  " 
or  "  systemic  "  circulation ;  the  path  from  the  right  ventricle  through  the 
vessels  of  the  lungs  to  the  left  auricle  is  called  the  "  lesser  "  or  "  pulmo- 
nary" circulation.  The  significance  of  this  double  circulation  lies  in  the 
facts  that  in  the  capillaries  of  the  tissues  of  the  body  the  blood  delivers 
up  food  and  oxygen  to  the  cells  and  receives  waste  products  from  them ; 
and  that  in  those  of  the  lungs  the  gaseous  waste,  carbonic  acid,  is  thrown 
off  into  the  air  to  be  expelled  in  the  breath,  while  the  blood  is  in  return 
charged  highly  with  oxygen.  The  left  side  of  the  heart  hence  carries 
purified  blood  charged  with  oxygen,  the  right  side  impure  blood  charged 
with  carbonic  acid  and  other  waste  matters.  These  latter  wastes  are 
removed  from  the  blood  capillaries  and  from  the  body  by  the  kidneys 
and  the  skin.  As  has  been  already  said,  the  food  is  received  into  the  cir- 
culation partly  in  the  capillaries  of  the  intestinal  wall  and  partly  directly 
into  the  veins  from  the  lymphatic  ducts. 

Thus  all  exchange  between  blood  and  living  protoplasm  takes  place 

through  the  capillary  walls.     In  harmony  with  this  function  these  walls 

consist  of  a  thin  membrane  made  up  of  flat  cells  joining 

Structure  one  anot;]ier  edge  to  edge,  and  allowing  a  ready  diffusion 

and  Function  .     .      ..       .  ,    ,  ,       n      .  .„.       ,-,,         ,      Al  .,, 

of  Vessels  °*  the  liquid  blood  plasma  (rig.  b).     As  the  capillaries 

pass  on  the  one  hand  into  the  arteries,  and  on  the  other 

into  the  veins,  the  walls  become  thickened  by  the  addition  of  a  layer  of 


muscle   outside 
outside  of  the 


of  the 
muscle. 


lining 


membrane  and  one  of  connective  tissue 
Both  these  layers  are  thicker  in  the  arteries 
than  in  the  veins,  and  in  the  former  the  con- 
nective tissue  is  highly  elastic.  Thus  the 
arteries  are  thick-walled,  active,  elastic  struc- 
tures, capable  of  altering  their  calibre  greatly, 
and  thus  regulating  the  amount  of  blood 
going  to  the  capillaries.  If  a  particular  cap- 
illary area  requires  a  large  quantity  of  blood, 
the  muscles  of  the  adjoining  arteries  relax 
and  the  arteries  dilate ;  if  less  blood  is  de- 
sired, the  muscles  contract  and  constrict  the 
supplying  arteries.  The  veins  are  thinner 
walled  and  passive,  and  are  in  brief  drainage- 
tubes  for  the  capillaries  and  the  tissues. 
The  arteries  are  thus  physiologically  more  interesting  than  the  veins. 
If  an  artery  be  cut,  the  thickness  and  stiffness  of  its  walls  cause  it  to 
stand  wide  open,  and  the  blood  gushes  freely  out ;  if  a  vein  be  severed, 
its  walls  collapse,  and  the  blood  hindered  in  its  flow  may  clot  more 
readily  and  less  loss  may  result.  Hence  wounding  an  artery  is  usually  a 
much  more  serious  and  dangerous  affair  than  wounding  a  vein.     In  this 


Fig.    6. — Capillaky    circulation 
in  the  web  of  the  frog's  foot. 


THE  HEART  AND  BLOOD-VESSELS.  99 

connection  it  is  interesting  to  recall  the  fact  that  as  a  rule  the  arteries 
lie  much  farther  from  the  surface  of  the  body  than  the  veins — a  most 
valuable  adaptation  of  Nature.      Exceptions  to  the  rule  are  the  radial 
artery  which  comes  near  to  the  surface  at 
the  wrist,  the  artery  at  the  temple,  and  a 
few  others.     But  the  veins  have  one  mechan- 
ism peculiar  to  themselves — that  is,  the  curi- 
ous   and    very    numerous    little    pouch-like 
valves  that  project  into  the  tubes  and  pre- 
vent any  back-flow  of  blood  toward  the  Cap-     Fio.  7.— Diagrammatic    section   or 

.,,         .  i  .       n  1  VEINS   WITH    VALVES. 

manes  when  any  influence,  sncli  as  pressure    ...  B   ,     ...    .,„„, . 

J  r  In  the  upper  figure  the  blood  is  sup- 

on  the  skin,  tends  to  hinder  the  venous  flow        posed  to  be  flowing  in  the  nor- 

/T^.        „.         -^            .-  ,,             .                  ,,  .            ,,     ,  mal  direction  from  C  (capillary) 

(.big.  7).       Even  it  the  veins  are   thin -walled  to  H  (heart);  in  the  lower  figure 

....          .1     .          ,           ,                 n          ,1     •  pressure  upon  the  surface  of  the 

and    inactive,  their  valves  do  not  allow  their  vein  has  temporarily  forced  the 

circulation   to   be  seriously  interfered   with        "°£  b(afuXy.)and  olosed  the 
by  extraneous  pressure. 

The  heart  in  its  embryonic  origin  is  a  simple  tubular  blood-vessel, 
and  in  some  of  the  lower  and  simpler  organisms,  such  as  the  worms  and 

the  tunicates,  it  retains  its  tubular  character  throughout 
Physiological  ]ife       But  {n  ^         wth  of  al]   hi   her  anima]s    jnclud. 

Anatomy  of  the      .  .        : =>  .  a-i-   a    u     v     u   • 

j2eart.  mS  man>  lts  simple  form  is  early  modified   by  its  being 

curved  upon  itself,  by  partitions  forming  within  its 
cavity,  by  valves  appearing  at  certain  places,  and  by  its  walls  becoming 
greatly  thickened  by  muscular  tissue.  It  thus  becomes  a  complicated 
muscular  pump,  a  part  of  the  circulatory  system  specially  modified  by 
Nature  for  the  purpose  of  propelling  the  blood  around  its  circuit.  For 
the  details  of  the  anatomy  of  the  heart  the  reader  is  referred  to  the  article 
on  The  Anatomy  of  the  Human  Body,  Fig.  27.  It  will  be  remembered 
that  the  organ  is  four-chambered,  comprising  two  thin-walled  upper  cham- 
bers, the  auricles,  and  two  thick-walled  lower  ones,  the  ventricles.  A 
partition  extends  the  whole  length  of  the  heart,  separating  the  auricle  and 
ventricle  of  the  right  side  from  those  of  the  left  side.  Each  auricle  re- 
ceives blood  from  veins,  and  opens  below  into  the  corresponding  ventricle. 
The  pulmonary  veins,  bringing  blood  from  the  lungs,  join  the  left  auricle  ; 
the  two  great  veins  that  bring  blood  from  the  rest  of  the  body,  the  supe- 
rior and  the  inferior  vense  cavse,  enter  the  right  auricle.  Each  ventricle 
opens  into  an  artery — the  right  one  into  the  pulmonary  artery  which  con- 
veys blood  to  the  lungs,  the  left  one  into  the  aorta,  which  is  the  largest 
artery  of  all,  and  supplies  with  blood  all  parts  of  the  body  except  the 
lungs.  The  course  of  the  blood  stream  has  been  mentioned  already,  and 
may  be  seen  readily  from  the  accompanying  diagram  (Fig.  5).  The  di- 
rection of  the  flow  is  determined  by  the  valves.  Of  these  there  are  two 
kinds  :  the  auriculo- ventricular  valves  at  the  opening  of  each  auricle  into 


100       PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN  HEALTH. 

its  ventricle,  and  the  semilunar  valves  at  the  origin  of  both  the  aorta  and 
the  pulmonary  artery.  The  former  allow  the  blood  to  pass  from  the  auri- 
cles to  the  ventricles,  but  not  to  return ;  the  latter  allow  it  to  flow  from 
the  ventricles  into  the  arteries,  but  not  to  return. 

The  movement  of  the  blood  is  caused  by  contractions  of  the  muscle 
in  the  walls  of  the  heart,  the  muscle  being  so  arranged  that  each  contrac- 
tion diminishes  the  size  of  the  heart  chambers,  and  the 
blood  is  thereby  squeezed  upon  and  forced  out.  The 
contractions  are  commonly  called  "  beats,"  and  are  performed  rhythmic- 
ally. In  the  newborn  child  the  heart  beats  at  the  rate  of  one  hundred 
and  thirty  to  one  hundred  and  forty  times  in  the  minute.  During  child- 
hood and  youth  the  rate  gradually  diminishes,  and  throughout  the  greater 
part  of  adult  life  remains  at  about  seventy-two.  The  events  of  each  beat 
in  brief  and  in  order  are  as  follows :  The  gradual  flowing  of  blood  from 
the  veins  into  the  auricles  and  from  these  into  the  ventricles ;  the  sudden 
short  contraction  of  both  auricles  for  the  purpose  of  overfilling  the  ven- 
tricles and  causing  the  auriculo-ventricular  valves  to  rise  upward  and  shut 
off  communication  between  auricles  and  ventricles ;  sudden  contraction 
of  both  ventricles  by  which  the  contained  blood  is  put  under  tension,  the 
semilunar  valves  are  pressed  open,  and  the  blood  is  shot  out  into  the  aorta 
and  the  pulmonary  artery  with  sufficient  force  to  drive  it  through  the 
arteries,  the  capillaries,  and  the  veins  back  again  to  the  opposite  auricle. 
Immediately  after  contraction  the  semilunar  valves  close,  each  heart  cham- 
ber relaxes  and  fills  with  blood  from  the  veins,  its  muscles  rest,  and,  in  a 
fraction  of  a  second,  have  recovered  energy  for  a  second  beat.  This  fol- 
lows, and  a  third,  and  a  fourth,  and  so  the  cycle  is  repeated  with  alterna- 
tions of  activity  and  of  rest,  of  systole  and  diastole,  throughout  the  life  of 
the  individual,  the  order  of  events  never  changing  unless  some  form  of 
"heart  disease"  interferes  with  the  working  of  this  most  beautiful  of  all 
animal  mechanisms. 

It  is  apparent  that  the  course  of  the  blood  along  the  arteries  is  inter- 
mittent, wave  after  wave  following  one  another  at  intervals  of  less  than  a 
second.  Each  wave  or  pulse  is,  of  course,  the  direct 
result  of  the  ventricular  beat,  and  hence  the  physician 
regards  the  pulse  as  one  of  the  best  indications  available  to  him  of  the 
condition  of  the  heart.  The  practised  ear  may  infer  much  also  from  the 
heart  sounds,  which  may  be  heard  readily  by  laying  the  ear  on  the  chest 
wall  over  the  heart.  These  are  two — a  longer,  faint,  low-pitched  tone,  due 
to  the  contraction  of  the  ventricles,  and  immediately  followed  by  a  short, 
higher-pitched,  abrupt  one,  caused  by  the  closing  of  the  semilunar  valves. 
If  the  parts  are  altered  by  disease  the  sounds  are  altered. 

The  smaller  arteries  and  the  capillaries  are  excessively  fine  tubes,  the 
diameter  of  some  of  the  latter  being  even  as  small  as  ^^oo  °f  an  inch,  the 


BEAT  OF  THE  HEART  AND  HEART  SOUNDS.       101 

diameter  of  a  red  corpuscle.  The  result  is  that  the  resistance  to  the  flow 
in  them  is  enormous,  and  the  blood  tends  constantly  to  accumulate  in  the 
arteries.  The  walls  of  the  arteries  are  thereby  put  under  great  tension, 
the  pressure  of  the  blood  within  them  is  great,  and  their  elasticity  is 
brought  into  play.  The  result  of  this  combined  capillary  resistance  and 
arterial  elasticity  is  that  by  the  time  the  blood  has  reached  the  smallest 
vessels  the  pulse  has  disappeared,  and  the  flow  is  continuous  in  both  the 
capillaries  and  the  veins.  Many  more  details  might  be  given  of  the  events 
of  the  blood  movement  were  there  space,  for  even  from  the  very  earliest 
times,  and  especially  since,  in  1628,  Harvey  gave  the  right  interpretation 
of  the  leading  facts,  and  since  Malpighi,  in  1661,  first  saw  with  his  micro- 
scope the  exquisite  and  wonderful  picture  of  the  blood  corpuscles  pick- 
ing their  way  through  the  tortuous  capillary  channels  in  the  wall  of  the 
frog's  lung,  the  circulation  has  been  a  favourite  study  with  all  schools 
of  physiologists.  Many  of  the  once  mysterious  facts  have  been  shown  to 
be  explicable  by  the  common  laws  of  mechanics.  Although  the  heart  is 
a  living  pump  and  the  vessels  living  tubes,  the  circulatory  system  pre- 
sents many  of  the  same  problems  as  are  presented  by  any  system  of  closed 
elastic  pipes  through  which  liquid  is  pumped.  Other  problems,  however, 
defy  the  attacks  of  the  mechanical,  physical,  or  chemical  physiologists, 
and  prove  that  the  experimental  laboratories  have  still  much  to  do. 

Of  these  latter  problems,  which  for  lack  of  a  better  word  may  be 
called  "  vital,"  two  may  here  be  mentioned — viz.,  the  causation  of  the 
heart  beat,  and  the  regulation  and  co-ordination  of  the  various  parts  of 
the  circulatory  apparatus.  The  beat  itself  is  an  excellent  example  of  what 
physiologists  are  wont  to  call  spontaneous  actions — *.  e.,  actions  which 
originate  within  the  tissue  itself,  and  do  not  require  a  stimulus  from  with- 
out to  set  the  tissue  going.  By  this  is  not  meant  a  causeless  action  ;  every 
action  has  one  or  more  causes,  but  the  causes  of  sponta- 
auseoj  neous  actions  are  to  be  sought  within  the  acting  part  it- 

self. The  beat  of  the  heart  is  a  spontaneous  action.  It 
has  been  abundantly  proved  in  lower  animals  and  even  in  the  higher 
quadrupeds  that  the  heart  when  removed  from  the  body  will  continue  to 
beat  even  for  hours  if  it  be  supplied  with  proper  nourishment  and  warmth, 
and  this  fact  no  doubt  would  apply  to  the  human  heart,  were  it  possible 
to  test  it.  The  impulse  to  the  beat  is  then  to  be  sought  in  the  heart 
itself,  but  in  what  tissue  ?  The  heart  contains  much  nervous  tissue,  con- 
sisting of  nerve  cells,  which  send  off  filaments,  the  nerve  fibres,  to  the 
cardiac  muscle  cells.  In  general  it  may  be  said  that  nerve  tissue  is  more 
inclined  to  spontaneity  than  muscle  tissue.  Within  the  heart,  then,  does 
the  impulse  to  beat  originate  in  the  muscle  cells  that  do  the  contracting, 
or  does  it  originate  in  the  nerve  cells  and  pass  from  them  along  the  nerve 
fibres  to  the  muscle  cells  ?     The  question  is  a  fundamental  one  for  physi- 


102      PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN   HEALTH. 

ologists,  and  its  answer  would  be  a  valuable  contribution  to  the  interest- 
ing subject  of  the  evolution  of  muscular  and  nervous  function.  In  the 
lower  and  simpler  animals  spontaneity  is  a  characteristic  of  almost  all 
kinds  of  cells  and  tissues ;  as  the  evolution  of  the  higher  animals  has  gone 
on,  gradually  the  tissues  have  become  less  spontaneous  in  their  actions 
and  more  dependent  upon  impulses  coining  to  them  from  the  nervous 
system,  until,  in  the  higher  animals,  the  work  of  the  body  is  largely  per- 
formed as  the  result  of  nervous  action.  In  accordance  with  this  the  nerv- 
ous system  retains  its  primitive  spontaneity  in  a  high  degree.  In  the 
hearts  of  invertebrates,  as  the  snail,  and  perhaps  some  low  vertebrates,  as 
the  frog  and  the  turtle,  the  impulse  to  beat  appears  to  originate  in  the 
cardiac  muscle  cells ;  in  the  higher  vertebrates  it  is  yet  unsettled  whether 
it  is  nervous  or  muscular  in  origin.  Why  the  heart  tissue  acts  rhythmic- 
ally is  another  inviting  subject,  not  yet  understood,  into  which  we  have 
here  not  time  to  go. 

Although,  as  we  have  seen,  the  heart  does  not  need  any  impulse  from 
outside  to  enable  it  to  continue  its  contractions  and  do  its  work,  yet  its 

contractions  are   always   regulated   and   controlled  by 
Nervous  Control  .  ,  ,.  ,-,       ,      .  ^  , , 

, .,    „     ,         nervous  impulses   coming  from  the  brain.     Jbrom  the 
of  the  Heart.  r  _  ° 

part  of  the  brain  lying  at  its  base  and  called  the  medulla 
oblongata,  situated  just  within  the  skull  at  the  back  of  the  neck,  nervous 
impulses  go  out  to  the  heart  along  certain  nerves.  Along  the  two  vagus 
nerves  (see  The  Anatomy  of  the  Human  Body)  may  go  impulses  which 
cause  the  heart  to  beat  more  slowly  or  more  weakly  than  before.  These 
come  from  the  so-called  cardio-inhibitory  centre  in  the  medulla,  and  if 
sufficiently  intense  they  may  cause  the  heart  actually  to  stop  beating. 
Along  the  sympathetic  nerves  may  go  impulses  that  cause  the  heart  to 
beat  more  rapidly  or  more  strongly  than  before  ;  these  come  also  from  an 
augmentor  or  accelerator  centre  situated  probably  in  the  medulla.  These 
two  centres  are  thus  antagonistic  in  their  action  on  the  heart.  They  are 
in  nervous  connection  with  other  parts  of  the  central  nervous  system  and 
thence  with  all  parts  of  the  body,  and  through  them  the  heart  is  delicately 
controlled  constantly  as  to  rate  and  force  of  beat,  so  that  its  work  is 
adapted  to  the  needs  of  the  body  at  every  moment.  As  examples  of  ex- 
treme activity  of  these  nerve  centres  may  be  mentioned,  first,  the  slowing 
or  actual  stopping  of  the  heart  by  a  sudden  heavy  blow  in  the  pit  of  the 
stomach,  or  by  a  sudden  shock  caused  by  a  piece  of  bad  news.  In  both 
cases  the  cardio-inhibitory  centre  is  stimulated  to  activity;  in  the  former 
through  nerves  going  up  from  the  stomach  to  the  medulla,  in  the  latter 
through  nerve  fibres  within  the  brain  itself,  extending  down  from  the 
consciously  acting  brain  centres  above  that  have  taken  cognizance  of  the 
bad  news.  In  both  cases  the  result,  as  stated,  is  slowing  or  stopping  of 
the  heart ;  the  fainting  that  usually  accompanies  is  a  secondary  result,  due 


NERVOUS  CONTROL  OF  THE  HEART  AND  ARTERIES.   103 

to  the  fact  that  the  weakened  heart  fails  to  pump  the  necessary  blood  to 
the  consciously  acting  brain,  and  unconsciousness  results.  Second,  the 
very  rapid  fluttering  of  the  heart  accompanying  mental  excitement  is  no 
doubt  due  to  excessive  stimulation  of  the  accelerator  centre,  and  thus  of 
the  heart,  by  impulses  coming  likewise  down  from  the  higher  brain  cells. 
A  little  consideration  will  show  that  slowing  of  the  heart  may  result 
theoretically  from  activity  of  inhibitor}'  nerves  or  from  the  cessation  of 
activity  of  accelerator  nerves ;  and  the  like  applies  vice  versa  to  accelera- 
tion. Physiology  has  not  yet  unravelled  all  the  mysteries  of  the  inter- 
actions of  these  two  antagonistic  nerve  influences.  It  may  be  mentioned 
here  incidentally  that  the  rapid  pulse  present  in  fever  is  probably  due  to 
the  hot  blood  stimulating  directly  the  heart  muscle  to  excessive  activity. 

The  blood  supply  to  the  various  parts  of  the  body  must  needs  vary 
constantly,  according  as  any  part  requires  more  or  less  blood  at  one  time 

than  at  another.     A  tissue  in  action  needs  more  blood 

Nervous  Control       ,1,1  ,  •  .    1  ■.  j  £      1 

, .,     ,  ,    .  than  the  same  tissue  at  rest,  because  it  needs  more  food 

of  the  Arteries. 

and  more  oxygen,  and  because  the  injurious  waste  prod- 
ucts of  its  activity  must  be  removed.  When  the  brain  thinks,  it  needs 
more  blood  than  when  it  sleeps  ;  when  the  digestive  glands  are  secreting, 
they  must  have  blood  in  abundance  ;  when  a  man  works  with  his  muscles, 
they  demand  an  extra  allowance  of  blood.  Obviously,  mere  alteration  of 
the  heart  beat  affects  the  general  blood  supply,  but  affects  all  parts  equally. 
Nature  has,  however,  evolved  an  efficient  method  of  varying  the  supply 
according  to  the  needs  of  the  individual  parts.  The  method  consists  in 
varying  the  calibre  of  the  artery  that  brings  blood  to  each  part.  Con- 
striction or  narrowing  of  an  artery  causes  the  quantity  of  blood  to  be 
diminished  ;  dilatation  or  widening  causes  it  to  be  increased.  The  calibre 
of  the  arteries,  like  the  action  of  the  heart,  is  regulated  through  special 
nerves  by  a  particular  part  of  the  brain.  Such  nerves  are  called  vaso- 
motor, from  the  fact  that  they  supply  the  muscles  or  motor  part  of  the 
arterial  walls.  The  vaso-motor  nerves  are  of  two  kinds,  quite  analogous 
in  their  functions  to  the  two  kinds  of  cardiac  nerves  :  the  vaso-constrictors 
have  the  power  of  causing  the  muscular  coat  of  the  arterial  walls  to  con- 
tract, and  thus  a  constriction  of  the  artery  results  (analogous  to  augmenta- 
tion of  the  heart  beat) ;  the  vaso-dilators  cause  relaxation  of  the  arterial 
muscle  and  hence  a  dilation  of  the  vessel  (analogous  to  cardiac  inhibi- 
tion). These  nerves  go  to  the  arteries  from  a  vaso-motor  centre  in  the 
,  medulla  oblongata.  This  part  of  the  brain,  like  the  cardio-inhibitory 
centre  near  which  it  lies,  is  affected  by  influences  coming  from  all  parts 
of  the  body,  and  its  actions  are  determined  by  the  nature  of  these  in- 
fluences. It  is  capable  of  controlling  the  calibre  of  each  artery,  and  thus 
of  constricting  or  dilating  small  or  large  vascular  areas.  Like  the  case  of 
cardiac  augmentation  or  inhibition,  arterial  constriction  and  dilation  may 


104      PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN   HEALTH. 

result  theoretically  not  only  from  direct  constricting  and  dilating  im- 
pulses, but  each  may  also  follow  from  the  cessation  of  impulses  leading  to 
the  opposite  activity.  Hence  the  mutual  interactions  of  these  two  in- 
fluences become  excessively  complicated  and  present  a  problem  not  yet 
wholly  solved.  As  examples  of  vaso-motor  actions  may  be  mentioned  the 
two  cases  of  blushing  and  becoming  pale,  due  in  the  one  case  to  arterial 
dilation,  and  in  the  other  to  arterial  constriction,  of  the  small  arteries  in 
the  skin  of  the  face.  The  exciting  cause  in  each  case  is  an  unusual  thought 
or  emotion  originating  in  the  brain  and  causing  nervous  impulses  to  pass 
down  to  the  vaso-motor  centre  in  the  medulla,  in  the  one  case  decreasing 
its  activity,  in  the  other  increasing  it.  Just  why  one  emotion  causes 
blushing  and  another  paleness  is  not  clear.  As  might  have  been  expected, 
of  all  the  blood-vessels  the  arteries  alone  are  known  to  be  markedly  con- 
trolled by  nerves.  The  vaso-motor  nervous  apparatus  and  the  cardiac 
nervous  apparatus  are  connected  within  the  medulla  and  work  in  harmony 
with  each  other.  Together  they  form  a  mechanism  of  remarkable  adapta- 
tion and  refinement. 

As  will  be  seen  from  the  article  on  Anatomy,  the  lymphatic  system  is 

comparable  in  a  general  way  with  the  capillary  and  the  venous  systems — 

i.  e.,  it  consists  of  capillaries  uniting  to  form  larger  ves- 

6 ay ,      '  sels,  and  these  in  turn  unite  into  two  laree  trunks.    The 

bystem.  \  ,  .    .       .       . 

capillaries  take  their  origin  in  irregular  spaces  among 

the  tissues.  (See  The  Anatomy  of  the  Human  Body,  Fig.  36.)  The 
lymphatics  receive  openings  from  such  large  cavities  as  those  of  the  ab- 
domen, of  the  chest,  and  of  the  pericardium.  At  intervals  they  open  into 
the  irregular  cavities  within  the  lymphatic  glands.  And  finally  the  two 
trunks,  the  thoracic  duct  and  the  smaller  right  lymphatic  duct,  open  into 
the  great  veins  at  the  root  of  the  neck.  The  walls  of  the  lymphatics  are 
not  unlike  those  of  the  capillaries  and  veins  in  structure,  but  they  are  ex- 
cessively thin.  Valves,  like  the  venous  valves,  are  very  abundant  in  the 
vessels.  The  lymph  not  only  fills  the  lymphatic  organs,  but  exists  also  in 
all  cell  spaces  and  interstices  of  the  tissues,  and  thus  bathes  the  living  cells 
much  more  intimately  than  does  the  blood.  The  lymph  may  be  regarded 
as  a  carrier  between  the  blood  and  the  living  cells,  all  food  and  all  waste 
matters  probably  having  to  pass  through  it  in  their  passage  between  the 
protoplasm  and  the  blood-circulating  system.  The  plasma  of  the  lymph 
is  blood  plasma  that  has  escaped  through  the  thin  walls  of  the  blood  cap- 
illaries into  the  spaces  in  the  surrounding  tissues  ;  the  corpuscles  of  lymph 
are  in  part  escaped  white  blood-corpuscles  and  in  part  new  cells  that  are 
formed  by  division  of  cells  within  the  lymphatic  glands.  The  lymph  thus 
originating  constantly  in  the  tissues  passes  into  the  lymphatic  capillaries, 
flows  constantly  along  the  vessels,  and  empties  itself  into  the  veins.  In 
all  vertebrates   below  the  mammals  a  varying  number  of  lymph  hearts 


THE  LYMPHATIC  SYSTEM.  105 

exist— simple  muscular  sacs  attached  to  various  lymphatic  vessels,  and 
capable,  like  the  blood  heart,  of  rhythmic  contractions.  No  such  organs 
are  known  to  exist  in  man  and  other  mammals.  The  movement  of  the 
lymph  is  due  to  several  agencies,  such  as  pressure  exerted  upon  the  vessels 
by  the  muscular  movements  of  the  body,  the  existence  of  a  lower  pressure 
in  the  veins  than  in  the  lymph-vessels  themselves,  and  possibly  rhythmic 
contraction  of  the  walls  of  the  vessels.  The  numerous  valves  prevent  any 
possibility  of  a  flow  in  the  wrong  direction.  Thus  both  structurally  and 
functionally  the  lymphatic  system  is  a  much  less  highly  specialized  appa- 
ratus than  the  blood  system.  The  latter,  with  its  efficient  means  of  pro- 
pulsion and  its  elaborate  nervous  mechanism  for  regulating  speed  and 
distribution,  is,  like  the  railway  system,  an  efficient  'and  rapid  carrier. 
But,  just  as  between  the  factory  and  the  railway,  or  between  the  latter 
and  the  consumer,  the  drayman's  cart  is  indispensable,  so  in  the  body,  be- 
tween the  place  of  digestion  and  the  blood,  or  between  the  blood  and  the 
living  cells,  the  lymph  finds  its  tasks.  The  lymphatic  system  and  the 
blood  system  together  form  a  most  efficient  distributing  and  collecting 
mechanism. 

We  have  thus  traced  the  food  from  outside  the  body  to  the  living  cells. 
Without  oxygen  the  cells  can  not  utilize  it.  We  have  now  to  consider  the 
source  of  the  oxygen. 

Section  III. 

RESPIRA  TION. 

The  lungs  and  other  respiratory  organs  have  a  twofold  function — that 

of  bringing  to  the  blood  the  oxygen  that  is  as  essential  to  life  as  is  the 

food,  and  that  of  removing  from 
Respiration  in       ^  b]ood  and  frQm  the  bo(J    water 

General.  . 

and  certain   waste  and   poisonous 

products,  mainly  carbonic  acid.  The  great  impor- 
tance of  the  whole  process  is  indicated  by  the  facts 
that  the  respiratory  organs  occupy  so  large  a  space 
in  the  body ;  that  the  right  ventricle  of  the  heart 
has  as  its  sole  function  that  of  supplying  blood  to 
them  ;  and  that  during  the  lifetime  of  the  individ- 
ual all  the  blood  in  the  body  must  pass  through  Flo  8_ termination  of 
them  once  in  every  twenty  or  twenty-five  seconds  ™™ZZ£t  beset 
— the  time  occupied  by  the  blood  in  making  the  ™«  AIR  SAC3-  (in- 
complete   circuit    of    the   body.      To   insure  rapid 

and  efficient  exchange  of  the  two  gases,  oxygen  and  carbonic  acid,  the 
blood  and  the  air  must  be  brought  into  as  close  proximity  to  each  other 
as  possible  ;  hence  we  find  the  lung  to  consist  mainly  of  innumerable 


106       PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN   HEALTH. 

small  air  sacs  (Fig.  8)  with  excessively  thin  walls  containing  a  little  elastic 
connective  tissue  lined  by  a  layer  of  flat,  thin  epithelium  cells,  and  loaded 
with  a  rich  network  of  fine  blood  capillaries.  The  air  sacs  are  continu- 
ous with  the  bronchial  tubes,  and  communicate  through  the  trachea  with 
the  outside  air  (Fig.  9).  The  blood  is  separated  from  the  air  by  the  thin 
epithelial   membrane   consisting  of   capillary  wall  and  wall  of  air  sac 


Fig.  9. — Trachea  and  lungs,  dissected  to  show  bronchial  tubes. 

1,  2,  larynx ;  3,  4,  trachea ;  5,  6,  bronchi ;  7,  8,  9, 10,  11,  bronchial  tubes,  a  few  only  being  shown  as 

far  as  their  terminations ;  12,  13, 14, 15,  surface  of  lungs.     (Sappey.) 

(Fig.  10).  Thus  the  conditions  needed  for  ready  diffusion  are  present. 
The  total  amount  of  air  surface  exposed  in  the  lungs  appears  to  be  more 
than  two  hundred  square  yards,  and  the  amount  of  capillary  surface 
more  than  a  hundred  and  fifty  square  yards.  The  blood  is  renewed 
constantly  by  the  circulation  ;  the  air  is  exchanged  constantly  by  the 
respiratory  movements  ;  and  the  result  of  the  interchange  between  the 
two  media  through  the  intervening  membranous  wall  is  that,  while 
impure  blood  and  pure  air  have  entered  the  lungs  by  their  respective 
channels,  pure  blood  and  impure  air  go  out  from  them. 


RESPIRATORY  MOVEMENTS.  107 

The  respiratory  movements  consist  of  those  of  inspiration  and  expira- 
tion. In  both  of  these  acts  the  lungs  are  passive  organs.  The  activity  re- 
sides in  the  muscles  of  the  chest  walls.  It  will  be  re- 
espira  ory  membered  that  the  lungs  are  inclosed  within  an  air-tight 
cavity,  the  thorax  or  chest,  which  is  bordered  at  the  top 
and  sides  by  the  ribs  and  the  intercostal  muscles,  and  below  by  the  dome- 
like muscular  partition,  the  diaphragm.  Contraction  of  the  external  in- 
tercostal muscles  raises  the  ribs,  pushes  the  sternum  or  breast  bone  out- 
ward, and  enlarges  the  whole  chest  cavity.  Enlargement  of  the  chest  may 
be  brought  about  also  by  contracting  and  thereby  lowering  the  diaphragm. 
The  inspiratory  act  consists  in  enlarging  the  chest  by  simultaneous  con- 
tractions of  both  these  muscles  with  the  assistance  of  other  muscles  of  the 
thoracic  walls.  The  walls  of  the  lungs  follow  passively  the  movements  of 
the  walls  of  the  air-tight  and  air-empty  chest,  and  the  capacity  of  the 
lungs  is  correspondingly  increased  ;  to  balance  this  the  air  rushes  in  pas- 
sively through  the  nostrils  or  mouth,  pharynx,  trachea,  and  bronchial  tubes. 
Thus  we  do  not  breathe  air  in,  as  our  sensations  might  mislead  us  to  be- 
lieve, through  any  action  exerted  upon  the  air  by  our  nostrils  or  lungs. 
When  we  wish  air  we  contract  our  diaphragm  and  the  muscles  of  our  ribs, 
and  air  must  come  in.  Both  sexes  use  both  muscles,  but  in  women  res- 
piration by  movement  of  the  ribs,  or  "  costal "  respiration,  predominates ; 
in  men  the  "  diaphragmatic  "  or  "  abdominal  "  method  is  more  promi- 
nent.    It  has  been  greatly  discussed  and  is  still  undecided  whether  this  is 

a  fundamental  sexual  difference  associated  with  the  func- 
Sexual  Differences     ..  .,.,,,..  ,    Al         .     .     , 

.     „      .    ..  tion  of  childbearing  in  woman,  or  whether  it  is  due  to 

in  Hespirahon.  c 

the  tightness  of  woman's  dress  about  the  abdominal  re- 
gion and  the  prevention  of  the  free  action  of  the  diaphragm.    The  expira- 
tory act  is  the  reverse  of  that  of  inspiration  :  the  muscles  cease  their  con- 
traction, the  ribs  and  the  diaphragm  return  to 
their  former  positions,  the  tension  on  the  lungs 
is  removed  ;  by  their  elasticity  the  lungs  return 
to  their  former  size,  and  the  excess  of  air  is 
squeezed  out.    Only  when  the  breathing  becomes 
laboured,  as  in   active  exercise,   or  when   from 
any  unusual  cause  there  is  danger  of  suffocation, 
does  expiration  become  a  muscular  act,  carried      FlG- 10.— Diagrammatic  view 

*  OF   AN   AIK   SAO. 

on  by  various  muscles  attached  to  the  walls  of     8)  lies  within  sac  and  point3 
the  chest  and  the  abdomen.     Respiration  is  a        to  epithelium  lining  wail; 

\  0,  partition  between  two  ad- 

rhythmic,  ordinarily  unconscious  action,  repeated        jaeent  sacs,  in  which  run 

.    ,  .  ,  capillaries ;    c,    elastic    con- 

on  an  average  seventeen  to  eighteen  times  in  the        nective  tissue.    (Huxley.) 
minute  in  the  adult,  but  more  rapidly  in  children. 

At  each  respiration  about  thirty  cubic  inches  of  air  ("  tidal  air  ")  passes 
into  and  out  of  the  air  passages  and  mixes  with  the  "  stationary  air  " 


108       PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN  HEALTH. 

(about  one  hundred  and  eighty  cubic  inches)  that  the  lungs  contain  at 
all  times.     The  actual  exchange  of  gases  between  the  tidal  and  the  sta- 
tionary air  takes  place  by  diffusion.     The  residt  of  the 

espimory        exchanee    is   that   the    expired   air    is   saturated   with 
Changes  in  Air.  °  ...... 

moisture,  is  warmer  than  the  inspired  air,  and  contains 

about  five  per  cent,  less  oxygen,  about  four  per  cent,  more  carbonic  acid, 

and  a  minute  quantity  of  obscure  deleterious  substances  of  unknown 

nature,  to  which  the  odour  of  the  breath  is  due. 

The  respiratory  muscles  are  not,  like  the  heart,  automatic  ;  they  need 

to  be  stimulated  for  each  contraction  ;  and  a  particular  part  of  the  brain 

has  been  specialized  to  originate  and  send  out  to  them 
Nervous  Control       ,,  .  ■  mi  •    ■    ,1  n   j  . 

.„      ...  the  necessary  impulses.     1  his  is  the  so-called  respiratory 

of  Respiration.  .  .  . 

centre  or  "  vital  spot,"  and  it  lies  in  the  medulla  ob- 
longata at  the  base  of  the  brain.  Its  presence  there  makes  this  portion  of 
the  brain  seem  so  important,  for  any  serious  injury  to  the  centre  stops 
respiration  and  thus  puts  an  end  to  life.  Hence  the  fatality  in  breaking 
the  neck.  The  nerve  cells  composing  the  centre  are  put  into  activity,  ap- 
parently, by  the  impure  venous  blood  circulating  about  them  ;  they  in- 
augurate an  inspiratory  impulse  and  discharge  it  along  the  intercostal  and 
the  phrenic  nerves  to  the  respiratory  muscles,  causing  the  latter  to  act. 
Exactly  how  the  regular  alternation  of  inspiration  and  expiration  comes 
about  is  not  wholly  explained,  but  the  centre  seems  to  be  regulated  in  its 
activity  by  nerve  impulses  coming  from  the  lungs.  It  is,  in  fact,  one  of 
the  most  sensitive  parts  of  the  nervous  system,  all  modifications  of  breath- 
ing that  take  place  in  laughing,  crying,  coughing,  sneezing,  hiccoughing, 
sighing,  "  catching  one's  breath,"  muscular  exercise,  talking,  singing,  step- 
ping into  a  cold  bath,  etc.,  being  due  to  influences  altering  the  regular 
working  of  the  respiratory  centre.  The  peculiar  facial  expressions  and 
characteristic  vocal  sounds  that  accompany  laughing  and  crying  are  to  be 
distinguished  from  the  modified  breathing.  It  is  not  easy  to  conceive 
how  and  why  these  peculiarities  of  facial  expression,  sounds  and  breath- 
ing, which  are  evidences  of  pleasure  or  of  grief,  and  the  beginnings  of 
which  seem  to  be  found  in  animals  lower  than  man,  have  been  developed. 
The  changes  that  the  blood  undergoes  in  its  passage  through  the  lungs 
are  in  harmony  with  and  are  no  less  striking  than  those  of  the  air.  The 
blood  loses  to  the  air  contained  in  the  air  sacs  of  the  lungs 
Respiratory  gjx  ^0  g^i^  per  cent_  0f  carbonic  acid,  and  gains  from  it 
Changes  in  the  ,  c  . 

Blood  eight  to  twelve  per  cent,  of  oxygen  ;  it  comes  to  the 

lungs  purplish  in  colour  ;  it  leaves  them  bright  scarlet. 
The  gaseous  exchange  between  air  and  blood  takes  place  chiefly  by  the 
physical  process  of  osmosis  through  the  thin  cellular  membrane  separat- 
ing them ;  but  the  cells  of  this  membrane  may  possibly  act  like  gland 
cells  to  "  secrete  "  the  two  gases. 


EXCHANGE   OF  OXYGEN   AND   CARBONIC  ACID.  109 

The  change  of  colour  of  the  blood  is  interesting.  It  will  be  remem- 
bered that  the  red  colour  is  due  to  the  colour  of  the  haemoglobin  that 

exists  in  the  red  corpuscles,  and  also  that  the  red  cor- 
Colour  of  the  Blood.  ,  ,,  .  ,,  „,       ,  ,    ,  . 

puscles  are  the  carriers  of  oxygen.      Ihe  haemoglobin 

exists  in  the  body  in  two  forms — in  venous  or  impure  blood,  largely  as 
reduced  hmmoglobin,  and  in  arterial  or  pure  blood,  as  oxyhoemoglobin. 
Reduced  haemoglobin  contains  little  oxygen  and  is  purplish  in  colour ; 
oxyhemoglobin  contains  much  oxygen  and  is  scarlet  in  colour.  The 
haemoglobin  that  is  brought  to  the  lungs  is  in  the  reduced  form ;  it  greed- 
ily seizes  upon  the  oxygen  that  is  absorbed  through  the  capillary  walls  ; 
it  becomes  oxidized ;  and  the  colour  of  the  haemoglobin,  the  red  cor- 
puscles, and  the  blood  changes  accordingly  to  the  bright-red  tint. 

Besides  the  pulmonary  respiration,  slight  exchange  of  oxygen  and  car- 
bonic acid  takes  place  directly  through  the  skin.     This  method  of  breath- 
ing is  of  great  importance  to  some  of  the  lower  animals, 
Respiration  by  ,  „  ,  ,         .  .     . 

the  Skin  sl        as  *rogs  an0-  worms,  but  in  man  it  is  very  sub- 

ordinate. 

The  specific  respiratory  organs  work  for  the  body  as  a  whole.  In  the 
broad  sense,  however,  all  living  cells  are  respiratory,  since  they  take  in 
oxygen  and  give  out  carbonic  acid.  Such  a  process  is  often  spoken  of  as 
internal  or  tissue  respiration.  The  lungs  and  the  skin  mediate  between 
the  air  and  the  blood  and  lymph.  The  blood  and  the  lymph  are  car- 
riers between  the  respiratory  organs  and  the  living  tissues.      Charged 

with  oxygen  in  the  lungs,  the  blood  is  sent  throughout 
Internal  .  . 

„     ■    t-  the  body,  and  everywhere  in  the  capillaries  it  courses 

among  living  cells  that  require  oxygen.  The  oxyhemo- 
globin is  robbed  of  its  contained  gas  and  becomes  reduced,  while  the 
blood  changes  to  a  purplish  colour.  The  cells,  on  the  other  hand,  are 
constantly  giving  off  carbonic  acid,  and  this  by  diffusion  passes  readily 
into  the  blood.  The  lymph  has  in  tissue  respiration  a  function  analogous 
to  that  which  it  has  in  tissue  nourishment :  it  is  the  mediator  between  the 
blood  and  the  living  cells.  Hence  the  result  of  tissue  respiration  as  re- 
gards the  blood  is  exactly  the  reverse  of  that  of  pulmonary  respiration. 
The  respiratory  relations  of  the  pulmonary  and  the  systemic  circulatory 
systems  hence  appear  in  a  new  and  striking  light.  The  former  deals  with 
the  respiratory  needs  of  the  body  as  a  whole,  the  latter  with  the  respira- 
tory needs  of  the  living  particles  of  which  the  body  is  composed. 

In  considering  respiration  we  have  unavoidably  touched  upon  the 
excretion  of  one  waste  product — carbonic  acid.  The  other  wastes  may 
now  be  studied. 


110       PHYSIOLOGY:    THE  VITAL  PROCESSES  IN  HEALTH. 

Section   IV. 

EXCRETION. 

Leaving  for  the  present  the  consideration  of  their  sources,  we  may 
enumerate  the  chief  waste  products  of  protoplasmic  activity  as  follows  : 

Waste  Products.  Organs  of  Excretion. 
Gaseous.  .  .  .Carbonic  acid.  Lungs,  skin. 

Liquid Water.  Kidneys,  lungs,  skin. 

,  Urea  and  other  nitro-  (  Kidneys>  skin. 

Solid <      genous  wastes.  ( 

(  Inorganic  salts.  Kidneys,  skin. 

Protoplasm  is  unable  to  extract  energy  from  these  substances,  and 
their  presence  in  the  body  in  excessive  quantities  is  harmful.  Nature  has 
therefore  evolved  in  the  organs  of  excretion  refined  mechanisms  for  re- 
moving them  from  the  organism.  They  are  cast  out  from  the  living  cells 
into  the  blood,  and  are  transferred  by  the  circulatory  organs  to  the  organs 
of  excretion.     The  latter  will  now  be  considered. 

A.  THE   KIDNEYS. 

The  kidneys  are  the  most  highly  specialized  of  the  excretory  organs. 
Their  duty  is  to  manufacture  and  pass  out  the  urine.  Urine  is  a  clear 
yellow  or  brownish-yellow  liquid,  consisting  of  about 
96-5  per  cent,  of  water  and  3"5  per  cent,  of  dissolved 
solid  substances.  The  solids  are  numerous,  comprising  organic  bodies 
such  as  urea,  uric  acid,  and  creatinin,  and  inorganic  salts  such  as  various 
sulphates,  phosphates,  and  chlorides.  Their  variety  and  relatively  con- 
siderable quantity  indicate  the  great  importance  of  the  kidneys  as  purifi- 
ers of  the  blood  and  thus  of  the  body.  The  greatest  interest  centres 
in  the  organic  solids,  of  which  urea  is  the  most  abundant,  because  they 
represent  the  characteristic  waste  products  of  the  destruction  of  the 
important  proteid  substance.  The  special  peculiarity  of  urea  and  these 
other  products  is  the  presence  within  them  of  nitrogen  ;  hence  the  urine 
is  the  medium  through  whicli  the  all-important  nitrogen  leaves  the  body. 
The  average  quantity  of  urine  that  is  passed  in  twenty-four  hours  is  one 
and  a  half  quarts,  but  the  quantity  is  subject  to  great  variations,  depend- 
ing upon  the  weather,  the  character  of  the  food,  whether  much  water  has 
been  drunk,  and  the  occupation  of  the  individual. 

The  kidneys  are  highly  complicated  glands  whose  structure  is  spe- 
cially adapted  to  the  removal  from  the  blood  of  large  quantities  of  water, 
together  with  solid  substances.  They  consist  of  a  mass  of  minute  canals 
(the  uriniferous  tubules),  and  of  blood  capillaries,  inextricably  woven 
together.     Each  tubule   begins   in  an  enlarged  cavity  (the  Malpighian 


STRUCTURE  AND  FUNCTION  OF  THE  KIDNEYS. 


Ill 


capsule)  into  which  projects  a  tuft  of  capillaries,  the  glomerulus.  From 
the  capsule  the  tubule  takes  a  tortuous  course,  as  shown  in  the  accompany- 
ing figure  (Fig.  11),  unites  with  other  tubules,  and  finally 
approaches  the  surface  of  the  kidney,  where,  together 
with  all  the  other  tubules,  it  opens  into  the  dilated, 
funnel-shaped  beginning  of  the  duct,  the  ureter  (Fig.  12).  The  wall  of  the 
tubule  consists  of  a  single  layer  of  living  cells  varying  in  thickness  in 


Structure  of  the 
Kidneys. 


Fig.  11. — Diagrammatic  view 
of  course  of  uriniferous 
tubules  in  kidney. 

/,  Malpighian  capsule,  con- 
taining glomerulus  ;  II- 
VIII,  course  of  tubule ;  IX, 
opening  of  tubule  into  pel- 
vis of  kidney  at  apex  of 
pyramid  of  Malpighi  ;  k, 
outer  surface  of  kidney;  r, 
outer  or  cortical  substance  ; 
ff,p,  inner  or  medullary  sub- 
stance.  (Huxley.) 


Fig.  12. — Vertical  section  of  kidney.  (Some- 
what smaller  than  natural  size.) 

1,  2,  3,  4,  pyramids  of  Malpighi,  striations  repre- 
senting uriniferous  tubules  ;  5,  apices  of  pyra- 
mids, where  tubules  open ;  6,  cortical  sub- 
stance projecting  inward  between  pyramids 
and  containing  blood-vessels;  7,  dilated  end 
of  ureter,  called  pelvis  ;  8,  ureter.     (Sappey.) 


different  parts.  In  among  the  tubules  is  the  very  close  network  of  blood 
capillaries,  and  lymph  permeates  all  the  interstices.  The  tubules  are  thus 
bathed  by  the  circulating  fluids. 

Most  of  the  constituents  of  the  urine  exist  ready  formed  in  the  blood, 
having  been  cast  into  it  by  the  cells  outside  of  the  kidney.     The  process 

of  excretion  consists  in  a  discharge  of  these  substances 
Kidneys  through  the  cells  that  form  the  walls  of  the  tubules. 

Some  of  the  cells  apparently  have  the  power  of  manu- 
facturing and  casting  out  the  few  constituents  that  are  not  present  in  the 
blood.     In  its  mode  of  action  the  kidney  thus  seems  to  combine  the  more 


112       PHYSIOLOGY  :    THE  VITAL   PROCESSES   IN   HEALTH. 


clearly  physical  features  of  the  work  of  lung  cells  and  the  more  ob- 
scure secretory  activities  of  gland  cells,  as  represented  by  the  digestive 
glands.  The  urine  is  secreted  constantly,  and  trickles  along  the  tubules 
to  the  dilated  end  of  the  ureter ;  it  then  leaves  the  kidney  in  the  latter 
tube  and  passes  to  the  urinary  bladder  situated  in  the  pelvis.  Here  it 
accumulates  until  the  distention  of  the  bladder  gives  rise  to  a  desire  to 
micturate.  Nervous  impulses  from  the  brain  cause  the  muscular  walls 
of  the  bladder  to  contract,  and  the  urine  is  discharged  from  the  body 

through  the  urethra. 

B.  THE  SKIN. 

The  skin  performs  a  variety  of  duties.     It  protects  the  delicate  parts 
within  the  body ;  it  contains  organs  for  the  senses  of  touch  and  of  tem- 
perature ;  through  its  blood-vessels  it  regulates  the  tem- 
Ih"  %"S  perature  of  the  body  ;  and  it  contains  important  excre- 

tory organs.     The  latter  are  the  two  kinds  of  glands 
known  as  sudoriferous,  or  sweat,  glands  and  sebaceous  glands  ;  the  former 

produce  the  sweat,  the  latter  the 
oily  substance  found  on  the  surface 
of  the  body. 

Sweat  glands  are  simple  tubes 
the  secreting  portion  of  which, 
coiled  into  a  knot,  lies  just  be- 
neath the  skin  (Fig.  13).  The 
duct  passes  through  the  skin  and 
terminates  by  a  minute  opening 
upon  the  surface.     These  "pores" 


Fig.  13. — Vertical  section  of  skin. 
(Magnified  20  diameters.) 

1,  outer  layer  of  skin ;  1,  2,  cuticle  or 
epidermis ;  3,  4,  inner  layer  of  skin 
or  dermis ;  5,  subcutaneous  tissue ; 
6,  sweat  glands ;  7,  masses  of  fat, 
consisting  of  fat  cells ;  8,  9,  ducts 
of  sweat  glands.     ( Sappey. ) 


Fig.  14. — Surface  of  palm  of  hand. 

(Magnified  4  diameters.) 

2,  ridges  in  skin  bearing  (1)  openings 

of  ducts  of  sweat  glands.     (Sappey.) 


may  readily  be  seen  by  a  common  magnifying  glass  upon  the  fine  ridges 
in  the  palm  of  the  hand   (Fig.  14).     Sweat  is  a  colourless,  salty  liquid 


EXCRETORY  FUNCTIONS  OF  THE  LUNGS  AND  SKIN.       113 

consisting  of  water  and  slight  quantities  of  urea,  inorganic  salts  (espe- 
cially common  salt),  and  a  few  other  substances.  It  is  constantly  given 
off.  An  average  quantity  is  nearly  a  quart  in  twenty- 
four  hours,  but  the  amount  varies  greatly  with  the 
weather,  the  occupation  of  the  individual,  and  other  influences.  The 
perspiration  is  thus  seen  to  be  an  important  medium  of  loss  of  sub- 
stance from  the  body.  This  is  especially  evident  when  one  exercises 
vigorously ;  it  is  easily  possible  in  an  hour's  exercise  to  diminish  one's 
weight  by  a  pound.  The  more  one  perspires,  the  less  the  kidneys  ex- 
crete, and  vice  versa.  Hence  the  skin  is  more  active  in  summer,  the 
kidneys  in  winter. 

The  sebaceous  glands  lie  in  the  deeper  part  of  the  skin  and  open 

chiefly  into  the  depressions  in  which  lie  the  roots  of 

Sebaceous  Glands.       ,        ,     .  „,  ..  ,    ,  .      ,.   ,  , 

the    hairs.      Ihey   secrete  an  oily   substance    of    slight 

value  as  an  excretion,  but  of  use  in  preventing  the  skin  and  the  hair 

from  becoming  too  dry. 

C.   THE   LUNGS. 

The  expired  air  is  an  important  medium  of  loss  of  water  and  the  chief 
one  for  loss  of  carbonic  acid.  The  excretory  function  of  the  lungs  has 
been  considered  sufficiently  under  respiration. 

We  have  followed  the  oxygen  and  the  food — consisting  of  proteids, 
fats,  carbohydrates,  salts,  and  water — from  without  the  body  by  way  of  the 
digestive  organs  and  the  lungs  to  the  living  cells.  We  have  seen  that 
waste  matters  in  the  forms  of  carbonic  acid,  urea,  salts,  and  water  go  from 
the  living  cells  by  way  of  the  excretory  organs  to  the  exterior.  What 
takes  place  within  the  living  substance? 


Section   V. 

METABOLISM. 

If  we  could  answer  the  question  propounded  at  the  end  of  the  last  sec- 
tion we  should  know  what  life  is  ;  yet  we  are  unable  to  point  to  any  one 
of  the  millions  of  cells  in  the  human  body  and  say  that  we  know  all  the 
details  of  the  vital  process  that  takes  place  within  it.  The  difficulties  and 
complexities  of  the  problem  are  inconceivably  great,  but  perhaps  not  in- 
surmountable. We  can  measure  and  analyze  the  income  and  the  outgo  of 
the  body  ;  we  can  test  the  effect  of  different  foods  upon  the  general  metab- 
olism ;  we  can  observe  how  the  composition  of  the  different  organs  changes 
when  food  is  withheld  from  the  animal  for  a  time,  all  of  which  methods 
are  helpful.  But,  further  than  this,  chemical  investigation  is  revealing  to 
us  ever  more  clearly  the  steps  in  the  pathway  between  food  and  wastes ; 
10 


114       PHYSIOLOGY  :    THE  VITAL  PROCESSES    IN   HEALTH. 

we  are  approaching  a  knowledge  of  the  structure  of  protoplasm  and  of 
the  structural  changes  that  take  place  in  the  actively  working  cell ;  and 
recent  studies  of  the  action  upon  organisms  of  the  environment  and  of  ex- 
ternal agents — such  as  light,  heat,  electricity,  and  chemical  influences — are 
giving  us  a  deeper  insight  into  the  secrets  of  the  physical  basis  of  life. 
We  can  conceive  the  nutritional  changes  that  take  place  within  the  cells 
and  that  constitute  the  metabolic  process  as  consisting  of  a  building  up 
and  a  breaking  down.     Raw  material  in  the  form  of  food  that  is  rich  in 

energy  is  brought  to  and  absorbed  by  the  cells.  It  is 

Metabolism  in  ,,         ,     ,         .     n       .,      ',  ,.       •,  ,  .,    . 

„         ,  altered  chemically,  its  atoms  are  recombmed,  and  it  is 

General.  ■"  ' 

built  up,  probably  by  a  complex  series  of  steps,  into 
protoplasm,  its  energy  being  retained  in  a  latent  form ;  this  is  the  con- 
structive phase  of  metabolism,  called  andbolism.  Later,  the  protoplasm  is 
changed  chemically,  its  atoms  are  recoinbined,  and  it  is  broken  down, 
probably  by  a  complex  series  of  steps,  into  wastes,  its  energy  being  given 
off  in  the  form  of  mechanical  work  and  of  heat ;  this  is  the  destructive 
phase  of  metabolism,  called  katabolism.  Stated  in  these  words,  the  vital 
process  seems  simple  enough.  But  this  is  not  the  whole  story  ;  for,  in 
the  first  place,  not  all  the  food  is  built  up  into  protoplasm  before  it  is 
broken  down  into  wastes  ;  some  appears  to  be  changed  at  once  after  enter- 
ing the  tissues  and  to  be  immediately  cast  out  in  the  excretions ;  other  food 
is  stored  up  for  a  time,  to  be  used  subsequently  for  the  manufacture  of 
protoplasm  or  for  other  needs  of  the  body.  Fat  is  an  excellent  example 
of  a  substance  that  is  thus  stored  ;  it  is  not  living,  but  is  contained  within 
special  living  cells — the  fat  cells.  The  cells  of  the  liver  are  likewise 
loaded  with  a  variety  of  starch,  called  glycogen.  Fat  and  glycogen  con- 
stitute a  stock  of  reserve  material  upon  which  the  cells  may  draw  in  time 
of  need.  Further,  each  of  the  different  varieties  of  cells  has  its  own  spe- 
cial metabolic  peculiarities.  For  example,  the  digestive  glands  are  pecul- 
iar in  manufacturing  in  quantity  substances  (the  digestive  fluids)  that  are 
of  the  greatest  subsequent  value  to  the  body  ;  certain  of  the  brain  cells 
are  unique  in  the  fact  that  their  activity  is  accompanied  by  phenomena  of 
thought ;  the  muscles  seem  to  be  the  greatest  producers  of  the  body  heat. 
All  these  peculiarities  complicate  the  problem  of  metabolism  greatly,  and 
the  real  difficulty  comes  when  we  attempt  to  learn  the  details  of  the 
matter. 

The  one  fact  that  stands  out  above  all  others  is  the  fact  that  the  de- 
structive   or   katabolic    process   is    one    of    oxidation  ;    that  is,   the   gas, 
oxygen,  is  made  to  unite  with  the  other  elements — car- 

_   .,  ..  bon,  hydrogen,  nitrogen,  etc. — that  exist  in  the  food  or 

Oxidation.  '     J         ^      '  &      ' 

in  protoplasm.  Thus,  of  the  various  excreted  sub- 
stances, carbonic  acid  is  a  compound  of  oxygen  and  carbon  (in  the  pro- 
portions CO,)  ;  water  is  a  compound  of  oxygen  and  hydrogen  (in  the  pro- 


FOOD   STUFFS  AND  WASTES.  115 

portions  H„0)  ;  and  urea,  while  more  complex  in  composition  (CH.JST^O), 
is  undoubtedly  formed  by  oxidative  processes  from  the  still  more  complex 
proteids.  In  this  vital  process  of  oxidation  heat  and  mechanical  energy 
are  set  free  for  the  body's  use,  hence  the  body  is  warm  and  can  do  work  ; 
and  the  excretory  substances  are  therefore  largely  devoid  of  energy. 
Oxidation  is  nothing  more  nor  less  than  combustion  ;  the  burning  of 
wood  or  coal  or  illuminating  gas  is  oxidation.  The  burning  of  these  life- 
less substances,  therefore,  and  the  vital  processes,  are  fundamentally  of  the 
same  chemical  nature.  So,  too,  as  regards  energy,  coal,  when  burned  in 
a  furnace,  yields  heat,  mechanical  work,  and  light ;  protoplasm,  when 
consumed  in  a  living  body,  yields  heat,  mechanical  work,  and  in  certain 
animals,  such  as  the  firefly,  glowworm,  and  some  marine  forms,  even 
brilliant  light.  Such  a  parallelism  between  lifeless  and  living  substance 
is  interesting  and  suggestive. 

Another    striking   fact    is    that    the    living    tissues   do    not    convert 

unchanged  into  their  own  substance  the  substance  of  the  corresponding 

tissues  that  are  eaten  in  animal  food.     For  example,  the 

SiilteAbZptL.  fat  of  the  food  does  not  g°  directly  to  form  the  fat  of 
the  body ;  the  muscle  proteid  of  beef  or  mutton  that  we 
eat  does  not  form  directly  our  own  muscles,  nor  does  the  animal  starch — 
glycogen — come  directly  from  the  starch  of  bread  and  vegetables.  On  the 
contrary,  all  of  the  food  stuffs  are  worked  over  by  the  living  substance, 
are  resolved  into  other  substances,  and  these  other  substances  are  recom- 
bined  into  the  proteids,  the  fats,  and  the  carbohydrates  that  are  found  in 
living  matter.  One  practical  application  of  this  principle  is  found  in  the 
fact  that  the  most  certain  method  of  increasing  one's  weight  by  fattening 
is  not  by  eating  large  quantities  of  fat,  but  rather  by  living  largely  upon 
a  carbohydrate  diet  (starch  and  sugar),  since  it  has  been  found  by  experi- 
ment that  such  a  diet  leads  directly  to  the  formation  of  body  fat.  On  the 
other  hand,  to  reduce  the  fat  of  one's  body,  a  diet  rich  in  proteid  is  most 
effective. 

As  to  the  uses  of  the  various  food  stuffs,  it  may  be  said  that  proteid  in 
some  form  is  always  necessary  in  the  food,  since  it  alone  of  the  chief  food 
stuffs  contains  nitrogen,  and  nitrogen  is  an  important 
unive,    sm'V     constituent  of  protoplasm.     Proteid  is  the  chief  source 
of  the  elements  of  new  protoplasm,  and  hence  the  value 
of  meats,  eggs,  and  milk  as  articles  of  diet.     Proteid  also  gives  energy  to 
the  body,  but  it  is  an  expensive  food.     Fat  is  very  rich  in  energy,  and 
protects  the  proteid  substance  within  the  protoplasm  from  destruction. 
It  may  with  advantage  take  the  place  of  some  of  the  proteid  in  the  food  ; 
"  a  streak  of  fat  and  a  streak  of  lean"  is  not  without  physiological  justifi- 
cation.   Carbohydrates  play  a  role  similar  to  the  fats  in  supplying  energy. 
They  are  a  cheaper  food  and  are  easily  digested.     The  part  that  is  played 


116       PHYSIOLOGY:    THE  VITAL  PROCESSES  IN  HEALTH. 

in  metabolism  by  inorganic  salts,  of  which  common  table  salt  may  be 
taken  as  the  type,  is  not  known.  The}'  exist  in  all  the  tissues,  and  they 
are  constantly  leaving  the  body  in  the  urine  and  the  sweat.  Depriving 
animals  of  salt  brings  on  weakness  and  even  paralysis  ;  yet  most  salts  are 
not  sources  of  energy.  All  that  is  known  upon  the  subject  may 
be  summed  up  in  the  statement  that  protoplasm  will  not  continue  to  do 
its  work  without  salt.  Hence  salt  is  needed  in  the  food  to  balance  the 
loss  through  the  excreta  and  thus  to  maintain  a  constant  supply  in  the  liv- 
ing substance.  Nor  is  water  a  source  of  energy.  Its  presence  every- 
where in  the  body  facilitates  the  chemical  reactions,  and  all  substances  in 
passing  from  one  part  of  the  organism  to  another  must  be  dissolved  in  it. 
It  constantly  leaves  the  body,  partly  for  the  purpose  of  regulating  the 
body  temperature,  as  we  shall  see  later,  and  partly  for  the  purpose  of  car- 
rying off  the  solid  part  of  the  excreta.  Without  it  protoplasm  dies,  and 
hence  its  necessity  in  the  food. 

Much  attention  has  been  given  to  the  determination  of  a  diet  most 
suitable  for  the  average  man,  and  it  is  now  known  with 
reasonable  accuracy  what  such  a  man  requires  for  his 
daily  needs.     A  fair  division  of  the  various  food  stuffs  is  given  below  : 

Proteids 4  oz. 

Fats 2  oz. 

Carbohydrates 17  oz. 

Salt 1  oz. 

Water nearly  3  qts. 

It  is  not  a  simple  matter,  however,  to  proceed  from  such  a  diet  to  the 

preparation  of  a  suitable  menu,  for,  although  the  proportions  of  food 

stuffs  in  the  common  foods  are  known  (Fig.  15),  it  does  not  follow  that 

all  the  food  stuffs  taken  into  the  mouth  are  absorbed  by  the  tissues.     The 

digestibility  of  the  foods  is  an  important  factor  too  often  overlooked  by 

rich  and  poor  alike.     A  liberal  allowance  of  beefsteak  will  not  yield  to 

the  body  four  ounces  of  proteid  unless  it  be  so  prepared  that  the  digestive 

organs  can  deal  with  it.     The  digestibility  of  the  various  foods  has  not 

yet  been  determined  with  sufficient  accuracy  ;  the  problem  is  a  difficult 

one,  complicated  as  it  is  by  the  idiosyncrasies  of  individuals  and  by  the 

infinite  varieties  of  method  used  in  the  preparation  of  foods. 

Scientific  investigation  and  the  common  experience  of  mankind  point 

to  a  combination  of  animal  and  vegetable  foods  as  the  most  suitable  diet. 

Meat  and  eggs  are  especially  characterized  by  the  presence  of  proteid  in 

a  concentrated  form ;  to  obtain  the  needed  amount  of 
Vegetarianism.  .    .  ,   .  ,    ,  ,  ,  n  ,, 

proteid  from  vegetables  would  require  the  consumption 

of  an  excessively  large  quantity  of  food.      A  strictly  vegetarian  diet 

throws  upon  the  digestive  organs  of  man  an  excessive  amount  of  labour. 


THE  DIGESTIBILITY  OF  FOODS. 


117 


This  is  not  so  in  the  case  of  herbivorous  animals,  since  in  them  the  teeth 
are  specially  modified  for  grinding,  the  alimentary  canal  is  relatively  long 
and  large,  and  the  digestive  processes  are  adapted  to  their  tasks.  In  man, 
however,  the  teeth  are  evidently  degenerating,  and  the  alimentary  canal 
is  becoming  reduced  in  capacity,  the  evident  outcome  of  which  in  time 


Beef. 

Pork. 

Fowl. 

Fish. 

Egg. 
Cow's  milk. 
Human  milk. 


Animal  Foods. 

Explanation  of  the  signs. 

Ullllllllllllll'ilill 


JProteids.    Albuminoids.    N-free  org.  bodies.        Salts. 

_.. .. ..  ... 


55 


■  « 

■  ' 


73 


73.5 


89 


Vegetable  Foods. 

Explanation  of  the  signs. 


,„al°-« 


0.4 


Proteids 


Wheaten-bread. 

Peas. 

Bice. 

Potatoes. 

White  Turnip. 

Cauliflower. 

Beer. 


Digestible       Non-digestible  Salts. 

N-free  organ  bodies. 


is    m 


00.5 


00 


6.5: 


I" 

J  2.5 

■  " 
]1 


0.5 


0.2 1 


0.5 


Fig.  15. — Composition  of  some  common  foods. 
Nitrogen-tree  organic  bodies  include  both  fats  and  carbohydrates. 

will  be  the  necessity  of  less  bulky,  more  easily  masticated,  and  more  easily 
digested  food.  The  misguided  vegetarians,  in  their  endeavours  to  make 
man  herbivorous,  forget  that  that  stage  in  his  career  was  passed  ages  ago, 
that  his  body  is  no  longer  fitted  for  it,  either  anatomically  or  physio- 
logically, and  that  stemming  or  turning  back  the  tide  of  evolution  is  not 
without  difficulties. 


118       PHYSIOLOGY  :    THE  VITAL  PEOCESSES   IN    HEALTH. 

The  two  chief  modes  of  manifestation  of  the  energy  of  the  body  are 
through  mechanical  work  and  through  heat.     These  may  now  be  consid- 
ered.    Mechanical  work  is  performed  by  the  muscles, 

nergy  of  ^^  corriprjses  ap  work  done  by  the  hands  and  arms,  by 

Mechanical  Work.  r  _  ... 

the  legs  in  walking,  by  the  trunk  in  lifting,  by  the  lar- 
ynx in  speaking,  etc.  It  is  comparatively  easy  to  measure  approximately 
the  amount  of  work  performed  by  a  man  in  a  given  task.  It  was  sup- 
posed formerly  that  the  energy  for  muscular  work  was  derived  solely 
from  the  nitrogen-containing  proteid  of  the  food  and  of  the  tissues.  If 
this  were  so,  and  since  urea  is  produced  by  the  destruction  of  proteid,  the 
quantity  of  urea  given  off  from  the  body  would  be  proportional  to  the 
amount  of  work  done.  Elaborate  investigations  made  upon  the  pedes- 
trian "Weston,  and  other  individuals,  in  order  to  test  the  question,  have  re- 
futed the  old  idea  and  have  shown  that  the  main  source  of  the  energy  of 
muscular  work  is  not  the  proteids,  but  rather  the  fats  and  the  carbohydrates. 
The  hunger  that  follows  labour  does  not,  therefore,  require  for  its  satis- 
faction an  increase  of  expensive  proteid  food.  About  one  fifth  of  the 
total  energy  introduced  into  the  body  by  the  food  appears  again  in  the 
form  of  mechanical  work,  the  remaining  four  fifths  taking  the  form  of 
heat.  At  first  thought  this  would  seem  to  indicate  that  the  muscle  is  a 
poorly  constructed  machine.  But  a  steam  engine  is  able  to  employ  for 
work  only  about  one  tenth  of  the  total  energy  of  the  coal,  the  heat  that  is 
lost  carrying  off  the  other  nine  tenths.  The  human  body  as  a  machine 
for  transforming  energy  is,  therefore,  much  superior  to  the  steam  engine. 
Perhaps  no  fact  in  all  human  physiology  is  more  striking  than  that 
the  body  of  man  is  warm,  and  constantly  warm  during  all  seasons,  even 
though  the  surrounding  temperature  may  be  excessively  low.  In  this  re- 
spect man  is  like  all  the  mammals  and  birds  and  differs  from  all  other 
animals.  The  terms  "  warm-blooded  "  and  "  cold-blooded  "  tell  a  part  of 
the  truth  only ;  "  warm-bodied  "  and  "  cold-bodied  "  are  equally  applicable. 

Yet  a  still  more  correct  designation  of  the  two  classes  of 
Body  Temperature.  ,     .     , ■,  „  ...  j      £      i 

animals  is  that  of  constant  temperature  and  of  change- 
able temperature.  An  adult  man's  body  has  a  temperature  of  about  98"6° 
Fahr.,  and  normally  varies  rarely  more  than  a  degree  above  or  below  that 
point.  A  frog's  body  can  not  be  said  to  possess  a  normal  temperature.  It 
is  always  cold  to  the  touch,  but  varies  within  very  wide  limits,  depending 
upon  the  temperature  of  the  surrounding  air  or  water.  Heat  is  produced, 
in  all  animal  and  plant  bodies,  but  is  dissipated  at  once  to  the  surround- 
ings in  all  organisms  except  the  warm-blooded  animals.  The  heat  comes 
from  the  oxidation  or  burning  of  the  food  and  body  substance,  and  is  de- 
rived from  all  three  chief  classes  of  food  stuffs.  Its  production  is  a  fun- 
damental property  ofprotpplasm,  and  takes  place  wherever  living  substance 
exists.     Hence  all  organs  and  tissues  yield  heat ;  but  the  muscles,  form- 


PRODUCTION  AND  LOSS  OF  HEAT.  119 

ing  as  they  do  so  large  a  proportion  of  the  bulk  of  the  body  and  being 
actively  metabolic  organs,  are  the  greatest  heat  producers.  The  more 
active  an  organ  is,  the  hotter  it  becomes.  The  blood,  while  producing 
little  heat,  performs  the  indispensable  role  of  equalizing  the  body  tem- 
perature. Receiving  its  own  warmth  from  the  tissues  through  which  it 
courses,  it  warms  the  more  sluggish  parts,  and  in  turn  cools  those  whose 
temperature  tends  to  become  dangerously  high.  "While  heat  is  produced 
constantly,  it  is  as  constantly  being  lost.  We  warm  our  clothing  and 
whatever  we  come  in  contact  with  that  is  of  a  lower  temperature  than 
the  body ;  the  air  that  we  breathe  out  is  warm  ;  the  excretions  are  warm ; 
much  latent  heat  goes  off  in  the  evaporating  sweat.  Of  all  these  path- 
ways of  loss,  the  skin  is  the  chief  one,  eighty  per  cent,  of  the  lost  heat 
leaving  the  body  through  it. 

As  has  been  seen,  a  warm-blooded  animal  differs  from  a  cold-blooded 
animal  in  the  fact  that  the  temperature  of  the  body  of  the  latter  changes 

with  that  of  the  surroundings,  while  that  of  the  former 
Regulation  of  .  ,■■,-,  ,        m,  „     ,  .      ,.„. 

Body  Temperature.  remains  practically  constant.  The  cause  of  this  differ- 
ence lies  in  the  fact  that  in  the  warm-blooded  organism 
both  the  production  and  the  loss  of  heat  are  carefully  controlled  by  the 
nervous  system.  As  regards  production  of  heat,  the  lower  the  tempera- 
ture of  the  air  the  more  heat  the  body  produces,  partly  through  invisible, 
obscure  metabolism,  partly  through  visible  muscular  movements,  such  as 
shivering  ;  on  the  other  hand,  the  higher  the  surrounding  temperature, 
the  less  active  are  the  muscles.  As  regards  loss  of  heat,  the  lower  the 
temperature  of  the  air,  the  less  blood  goes  to  the  skin,  and  hence  the  less 
heat  radiates  from  the  surface  ;  on  the  contrary,  the  higher  the  tempera- 
ture of  the  air,  the  more  the  cutaneous  vessels  dilate  and  allow  the  heat  of 
the  blood  to  pass  off  ;  at  the  same  time  the  body  perspires  and  gives  out 
abundant  latent  heat  in  the  sweat.  Thus  the  vaso-motor  nerves,  the  se- 
cretory nerves  of  the  sudoriferous  glands,  and  other  nerves  are  employed 
for  heat  regulation,  and  their  various  activities  are  brought  into  harmony 
through  the  central  nervous  system.  In  the  cold-blooded  organism  no 
such  regulating  mechanism  exists. 

In  discussing  metabolism  the  liver  must  not  be  overlooked,  since  it 
plays  so  important  and  such  various  roles  in  nutrition.     "We  shall  here 
enumerate  its  chief  functions.     The  liver  produces  bile, 
the  Liver  khe  use  °^  which  m  the  digestion  of  fats  we  have  dis- 

cussed. In  addition  to  this  digestive  property  bile  con- 
tains several  complex  substances  which  must  be  regarded  as  waste  prod- 
ucts of  protoplasmic  activity,  and  are  passed  off  from  the  body  with  the 
undigested  food  matters  ;  hence  the  liver  is  an  organ  of  excretion.  The 
liver  cells  contain  abundant  glycogen,  or  animal  starch,  which  is  regarded 
as  a  reserve  stock  ;  hence  the  liver  is  a  storehouse  of  carbohydrates. 


120       PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN  HEALTH. 

Lastly,  the  liver  seems  to  be  the  chief  organ  in  which  take  place  the  final 
processes  in  the  manufacture  of  urea ;  the  raw  material  for  excretion 
comes  in  the  blood  from  the  various  organs  to  the  hepatic  cells,  the  cells 
transform  them,  and  the  finished  product,  urea,  is  transferred  to  the 
kidneys  for  elimination.  With  such  a  multiplicity  of  functions  it  is 
not  surprising  that  "  liver  complaints  "  form  so  large  a  proportion  of 
human  ills. 

The  spleen,  the  thyroid  body,  the  suprarenal  bodies,  and  the  thymus 
have  apparently  important  metabolic  functions,  but  their  exact  roles  are 
little  known.     At  present  they  are  being  actively  studied. 

Our  story  oftiutrition  is  completed.  It  is  but  the  preliminary  to  a 
study  of  what  may  be  called  by  some  the  higher  functions  of  the  body. 
To  these  we  now  turn. 


CHAPTER   II. 
MOTION. 

Section   I. 

MUSCLE  IN  GENERAL. 

Next  to  producing  heat,  the  chief  mode  in  which  the  body  employs 

its  stock  of  energy  is  by  doing  mechanical  work.     The  physical  sign  of 

mechanical  work  is  motion.     Its  manifestation  is  most 

„         7  evident  in  the  day  labourer.     But  the  professional  man 

Creneral.  •>  r 

and  those  who  are  known  technically  as  brain  workers 
are  not  simply  heat  producers.  Aside  from  the  ordinarily  invisible  in- 
voluntary movements  of  the  organs  and  the  visible  voluntary  movements 
of  the  body,  without  which  no  man  passes  through  each  succeeding 
twenty -four  hours,  the  man  who  thinks  gives  his  thoughts  to  the  world 
in  writing  or  speaking  or  acting,  all  of  which  are  processes  of  movement. 
The  organs  of  mechanical  work  are  the  muscles.  A  muscle  is  made  up 
of  muscle  tissue  and  this  in  turn  of  muscle  cells.  Muscle  cells  always 
have  one  axis  considerably  longer  than  the  other  two,  from  which  fact 
they  are  often  called  fibres,  and  the  essence  of  their  activity  consists  in 
their  power  of  contracting  or  shortening  in  the  direction  of  the  long 
axis ;  in  this  process  movement  of  attached  parts  is  caused.  This  power 
of  contractility  is  one  of  the  fundamental  attributes  of  protoplasm ;  it  is 
possessed  by  the  living  parts  of  plants  and  by  the  one-celled  animals.  As 
the  evolution  of  animal  life  in  past  ages  went  on  and  one-celled  animals 
gave  rise  to  many-celled  animals  with  a  variety  of  functions,  the  ability 
to  contract  became  progressively  stronger  in  some  cells  than  in  others. 


THE  ORGANS  OF   MECHANICAL  WORK. 


121 


Structure  and 
Varieties  of 
Muscle. 


In  these  contractile  structure  and  function  went  on  developing  together, 
they  became  more  and  more  perfected  as  organs  of  movement,  and  they 
gave  rise  finally  to  the  highly  specialized  muscle  cells  that  organisms 
possess  to-day. 

When  we  look  over  the  whole  animal  kingdom 
we  recognise  a  very  great  variety  of  muscle  tissue, 

from  the  simple  cells  of  the  jelly 

fishes,  where  only  a  part  of  the 

cell  can  be  called  muscular,  to 

the  highly  differentiated  muscle 
fibres  of  actively  moving  organs  like  the  wings 
and  legs  of  insects  and  the  limbs  of  the  higher 
animals.  Notwithstanding  this  variety,  muscle 
cells,  especially  in  man  and  other  vertebrates,  may 
be  grouped  into  three  chief  classes  that  are  distin- 
guished from  each  other  both  structurally  and 
functionally.  These  are  known,  respectively,  as 
smooth,  striped,  and  cardiac  muscle  cells  or  fibres. 
Smooth  or  unstriped  muscle  fibres  are  so  called 
because,  in  distinction  from  the  other  two  varie- 
ties, they  are  not  cross-striped  (Fig.  16).  They 
are  usually  spindle-shaped  ;  they  are  most  common 
in  tubular  organs,  occurring  in  the  walls  of  the 
alimentary  canal,  the  arteries  and  the  veins,  the 
ducts  of  glands,  the  trachea,  and  in  general  in  those  parts  of  the  body, 
except  the  heart,  that  are  capable  of  involuntary  movement  only.  They 
are  bound  together  by  connective  tissue  into  muscular  coats  encircling- 
the  tubes  in  whose  walls  they  lie,  and  by  their  contraction  they  constrict 
the  tubular  organs.  They  are  not  under  the  control  of  the  will.  They 
are  the  most  primitive  of  the  three  kinds  of  muscle  substance.  Their 
action  is  slow,  as  is  indicated  by  the  writhing,  wavelike  movements  of 
the  stomach  and  the  intestine  during  digestion.  Striated  or  striped  mus- 
cle fibres  are  so-called  because,  when  examined  with  the  microscope,  they 
appear  indistinctly  cross-striped  with  alternate  lighter  and  darker  bands 
(Fig.  IT).  They  are  very  long,  delicate,  threadlike  cells,  and  their  proto- 
plasm is  highly  complicated  in  structure.  They  are  usually  under  the 
control  of  the  will,  and  form  the  flesh  or  meat  of  the  body — that  is,  the 
muscles  of  the  arms,  the  legs,  the  head,  and  the  trunk.  Each  muscle 
consists  of  a  mass  of  innumerable  fibres  bound  together  by  connective 
tissue  into  bundles,  and  is  attached  usually  to  bones  by  means  of  tendons. 
Striped  muscle  is  the  most  highly  specialized  kind  of  muscle  tissue.  It 
is  capable  of  very  quick  action,  as  is  shown  by  the  rapidity  with  which 
one  can  strike  a  blow  or  play  a  piano.     Cardiac  muscle  is  intermediate 


Fig.  16. — Unstriped  muscle 
fibres  of  man.  (Magnified 
200  diameters.) 

1,  nuclei  of  fibres;  2,  fibres 
in  mass;  3,  isolated  fibres; 
i,  4,  two  fibres  joined  to- 
gether at  5.    (Sappey.) 


122       PHYSIOLOGY:    THE  VITAL  PROCESSES  IN  HEALTH. 


Fig 


(Magnified 


structurally  between  the  other  two.  It  consists  of  short,  compact,  indis- 
tinctly striped  cells.  It  occurs  only  in  the  heart,  and,  as  we  have  learned 
in  studying  the  heart  beat,  contracts  spontaneously,  involuntarily,  and 
rhythmically. 

For  the  past  fifty  years  muscle  has  been  a  fascinating  and  fruitful 
field  of  physiological  research.     Many  of  the  most  interesting  and  funda- 

„  mental  problems  of  general  proto- 

plasmic action  have  been  and  may 
be  studied  here  more  successfully 
than  in  other  tissues ;  not  only  the 
laws  of  vital  movement,  but  funda- 
mental questions  such  as  antoma- 
ticity,  irritability,  rhythm,  animal 
electricity,  and  the  chemical  phe- 
nomena of  life.  A  large  variety  of 
delicate  and  valuable  apparatus  has 
been  devised  for  the  exact  investi- 
gation of  these  various  problems. 
The  study  of  striped  muscle  has 
yielded  the  most  information,  and 
our  present  discussion  will  be  lim- 
ited to  this.  As  has  been  stated, 
the  essence  of  muscular  action  con- 
sists in  the  power  of  the  muscle 
fibres  to  contract.  In  cold-blooded 
animals,  such  as  the  frog,  the  muscles  retain  their  contractile  power — that 
is,  remain  living — long  after  the  animal  has  been  hilled,  hence  it  is  easy 
in  such  animals  to  study  muscular  action.  During  life  the  muscles  are 
made  to  contract  through  impulses  coming  to  them  along  the  nerves  from 
the  brain  or  spinal  cord.  After  death  in  cold-blooded  animals  they  may 
be  stimulated  to  activity  by  electric  shocks,  heat,  pinching,  or  by  certain 
chemicals  applied  either  to  the  muscles  directly  or  to  their  nerves.  For 
each  stimulation  the  muscle  gives  a  single  twitch  or  contraction,  during 
which  it  shortens  and  becomes  thicker  and  harder,  and  then  immediately 
relaxes  into  its  former  state.  The  whole  period  of 
activity  occupies  only  about  one  tenth  of  a  second,  yet 
during  this  moment  the  muscle  undergoes  profound  molecular  changes. 
Besides  the  mechanical  changes  spoken  of,  it  produces  heat  and  becomes 
warmer,  produces  carbonic  and  lactic  acids,  and  develops  a  considerable 
electric  current.  All  these  phenomena  indicate  what  great  metabolic 
changes  muscle  protoplasm  is  subjected  to  during  activity.  It  would  be 
interesting  to  trace  these  further,  but  it  would  take  us  beyond  our  present 
space.     In  life  it  is  probable  that  voluntary  muscle  rarely,  if  ever,  gives 


-Striped    muscle    fibres. 
250  diameters.) 
A,  piece  of  a  fibre  showing  cross-striations  and 
nuclei ;  B,  piece  of  a  fibre  with  cell-wall  (sar- 
colemma)  ruptured,  showing  tendency  of  fibre 
to  split  into  fibrillar.     (Sappey.) 


Action  of  Muscle. 


MUSCULAR  MECHANISMS,  ACTION  AND  TONE.  123 

single  isolated  twitches.  Each  contraction,  however  quick,  consists  of 
numerous  single  contractions  following  one  another  at  the  rate  of  about 
twelve  in  the  second,  and  becoming  fused  into  a  compound  contraction 
called  tetanus.  These  rapid  contractions  give  rise  to  a  dull  booming 
sound  which  is  emitted  by  the  muscle  and  may  readily  be  heard  by  in- 
serting the  tips  of  one's  fingers  into  the  ears  and  contracting  strongly  the 
muscles  of  the  arms.  In  health  the  muscles  always  seem  to  be  in  a  state 
,     „  of    slight    contraction,   or  "tone,"   which    accounts   in 

jjlUSCUlct'F    TOTVB 

great  part  probably  for  the  elasticity,  springiness,  and 
ready  muscular  response  of  the  athlete.  This  healthy  tone  appears  to  be 
due  to  nervous  impulses  coming  constantly  to  the  muscles  from  the  spinal 
cord.  It  is  noticeably  absent  in  ill  health.  Many  muscles  are  so  placed 
as  to  antagonize  the  actions  of  others.  For  example,  the  flexors,  which 
bend  the  arms,  legs,  fingers,  and  toes,  act  in  opposition  to  the  extensors 
which  straighten  the  same  parts ;  the  eye  is  closed  by  the  orbicularis  and 
opened  by  its  antagonist,  the  levator  of  the  upper  lid ;  and  the  delicate 
adjustments  of  the  parts  of  the  larynx  in  speaking  and  singing  are  due  to 
refined  balancing  of  opposing  muscles. 


Suction    II. 

SPECIAL  MUSCULAR  MECHANISMS. 

As  instances  of  special  motor  phenomena  we  have  already  mentioned 
the  digestive  movements,  the  beat  of  the  heart,  arterial  constriction  and 
dilation,  and  the  movements  of  respiration.  We  may  now  notice  briefly 
a  few  others. 

A.  LOCOMOTION. 

The  erect  posture  in  standing  or  sitting  requires  the  co-ordinated 
action  of  numerous  muscles  of  the  trunk  and  the  legs.  That  this  is  so  is 
evident  from  the  fact  that  the  body  collapses  whenever,  as  in  fainting, 
the  muscles  fail  to  receive  their  customary  stimulating  impulses  from  the 
central  nervous  system. 

Walking  is  a  complicated  muscular  act,  participated  in  by  a  large 
number  of  muscles  of  the  legs,  the  trunk  and  the  arms,  that  contract  in 
regular  sequence  (Fig.  18).  Each  leg  is  flexed  as  it  is  swung  forward 
like  a  pendulum  ;  it  is  then  straightened  and  serves  as  a  support  for  the 
swinging  trunk.  The  movements  of  the  trunk  are  peculiar.  It  alter- 
nately rises  and  sinks,  in  falling  forward,  and  sways  from  side  to  side 
as  the  centre  of  gravity  of  the  body  comes  now  over  the  right,  now  over 
the  left  foot ;  at  no  time  is  the  body  wholly  free  from  contact  with  the 
ground,  and  for  a  brief  interval,  when  the  feet  are  farthest  apart,  both 
feet  touch  it  at  the  same  time.     In  running,  the  sequence  of  events  is  so 


124       PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN   HEALTH. 

far  different  from  that  in  walking  that  at  one  time  the  body  is  entirely 
free   from  support.     The  recent  advances  in  the  art  of  instantaneous 


2 


Fig.  18. — Series  of  figures  from  instantaneous   photographs   to   illustrate  movements  in 

slow  walking. 

All  phases  of  movement  of  the  right  arm  and  leg  are  shown  in  figures  I  to  VI.  Arabic  numerals 
indicate  the  corresponding  positions  of  the  left  arm  and  leg ;  thus  position  III  of  the  right  side 
is  simultaneous  with  position  6  of  the  left  side.     (Marey.) 

photography  have  added  much  to  our  knowledge  of  the  mechanism  of 
bodily  movements. 

B.  FACIAL  EXPRESSION. 

Changes  in  facial  expression  are  muscular  phenomena,  due  to  contrac- 
tion of  the  facial  muscles  in  combinations  varying  with  the  various  emo- 
tions. This  is  shown  by  the  fact  that  it 
is  easy  by  electrical  stimulation  of  the 
muscles  through  the  skin  to  produce  arti- 
ficially in  an  individual  at  will  a  desired 
emotional  expression  (Fig.  19).  The 
anatomical  peculiarities  of  the  face  con- 
stitute the  features  and  give  a  certain 
set  to  every  countenance.  The  expres- 
sions are  physiological,  and  are  produced 
in  much  the  same  manner  in  different 
individuals.  In  his  book,  entitled  The 
Expression  of  the  Emotions  in  Man 
and  Animals,  Darwin  has  given  the 
results  of  a  careful  study  of  expression. 
He  analyzes  into  their  various  muscular 
components  the  changes  accompanying 
joy,  grief,  despair,  love,  hatred,  anger, 
disdain,  contempt,  pride,  surprise,  fear, 
horror,  etc.  He  finds  the  origin  of  many  of  these  expressions  in  the 
lower  animals,  and  shows  how  they  have  gradually  become  habitual  and 


\     X 


Fig.  19. — Expression  of  extreme 
terror. 
Produced  artificially  by  stimulating  with 
electricity  the  muscles  of  the  forehead 
and  the  "jaws.  The  four  curved  rods 
are  the  stimulating  electrodes  laid  upon 
the  skin  over  the  muscles.  (From 
Darwin,  after  Duchenne.) 


CHANGES  IN   FACIAL   EXPRESSION. 


125 


innate  in  man.     Their  primary  purpose  was  not  to  reveal  emotional  states 
of  mind,  but  rather  they  were  either  of  some  direct  bodily  benefit  to  the 


_   -  „ 


: '"■'"■ 


Fig.  20. — Expression  of  various  emotions,  showing  characteristic  muscular  contractions. 

A,  pride  and  defiance ;  B,  helplessness ;  C,  childish  joy ;  D,  childish  grief.     (A  and  B  from 

Darwin  after  Duchenne.) 


individual  or  were  indirectly  the  effect  of  a  general  excitement  of  the 
nervous  system.     (See  Fig.  20.) 


126       PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN  HEALTH. 


C.  VOICE. 

The  production  of  voice  is  the  most  delicate  of  all  muscular  acts  of 
which  the  human  body  is  capable.  The  larynx,  the  organ  of  voice,  is  the 
modified  upper  end  of  the  trachea.  However  much  the  voice  may  seem 
to  arise  at  the  lips,  or  in  the  mouth,  or  in  the  chest,  it  is  only  modified  by 
these  parts,  and  the  sound  is  produced  in  the  larynx 
Physiological  only.  Considered  as  a  musical  instrument,  the  larynx  is 
L  r  nx  most  nearly  like  a  reed  instrument,  of  which  the  clario- 

net is  an  example  ;  but  the  resemblance  is  not  close.  In 
the  larynx  the  parts  that  correspond  to  the  reeds,  by  the  vibration  of 
which  voice  is  produced,  are  the  two  vocal  cords  (Fig.  22).  They  are  elas- 
tic membranes  that  extend  from  each  side  horizontally  toward  each  other 
into  the  cavity  of  the  hollow  larynx,  and  are  stretched  more  or  less  from 
before  backward.  They  do  not  meet  in  the  middle  line,  but  have  between 
them  a  chink  of  variable  width,  the  glottis,  extending  across  the  larynx 
from  front  to  back.  Each  cord  is  thickened  with  muscle  at  its  outer  part 
attached  to  the  walls,  but  its  free  edge  at  the  glottis  is  thin,  and  consists 
of  white,  tough,  elastic  connective  tissue.  Thus  the  air-passage  to  and 
from  the  lungs  is  obstructed  at  its  upper  end  by  this  horizontal  mem- 
branous partition,  with  a  passageway  for  air  between  its  two  halves. 
During  ordinary  silent  breathing  this  obstruction  is  slight,  for  then  the 
vocal  cords  recede  to  the  side  walls  of  the  larynx  and  the  glottis  is  wide 
open.  During  speaking  or  singing  the  cords  are  extended  in  toward 
each  other  and  the  glottis  is  reduced  to  a  mere  slit. 

Voice  is  the  sound  produced  by  the  rapid  vibration  of  the  thin  edges 

of  the  cords  as  the  air  rushes  between  them  in  expiration.     The  essential 

conditions  of  the  production  of  voice  are  that  the  cords 
Conditions  of  Voice.  .    ,  ,    .,     .         ,  ,    ,  .   , 

must   be  taut  and  their  edges  must   be  approximately 

parallel.  These  conditions  are  fulfilled  through  the  various  delicate 
muscles  acting  upon  the  cartilages  to  which  the  cords  are  attached,  or 
even  upon  the  cords  directly. 

The  natural  pitch  of  a  voice  depends  upon  the  natural  length  and  ten- 
sion of  the  cords.     Variations  in  pitch  are  produced  by  varying  either 
the  degree  of  tension  or  the  length  of  the  vibrating 

'       ,..     '      cord,  or  by  varying  both  together.     Thus,  for  the  low 
and  Quality.  '  •'  J      °m  n 

tones,  the  more  tightly  the  cords  are  stretched  the 
higher  the  note,  just  as  is  the  case  in  tuning  a  violin.  For  the  high  tones, 
the  cords  do  not  usually  vibrate  along  their  whole  length  ;  a  portion, 
usually  the  posterior,  is  "  stopped  "  by  the  cords  being  brought  into  con- 
tact with  each  other,  and  the  anterior  part  only  is  capable  of  acting  (Fig. 
22,  B),  just  as  in  playing  the  violin  the  pitch  is  regulated  by  placing  the 
finger  upon  the  string.     Loudness  of  voice  is  determined  by  the  strength 


QUALITY,   RANGE,   AND  PITCH  OF  THE  HUMAN   VOICE.    127 


of  the  outgoing  current  of  air.  The  quality  of  the  voice — that  by  which 
we  distinguish  one  voice  from  another  and  recognise  the  voices  of  our 
friends — depends  upon  the  make  and  the  age  of  the  individual  larynx,  its 
size,  the  quality  of  the  cords,  and  the  shape,  the  size,  and  the  mutual  re- 
lations of  the  accessory  vocal  organs,  such  as  the  mouth  and  its  parts,  the 
nose,  the  pharynx,  and  the  chest ;  in   like  manner  the  quality  of  tone 


256 


Soprano. 


1024 


171 


Contralto. 


684 


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Bass. 


342 


Control  of  the  Voice. 


128  Tenor.  512 

Fig.  21. — Tub  average  range  of  human  voices. 
c'  to  f  is  common  to  all  voices.     The  figures  indicate  the  number  of  vibrations  per  second  in  the 
corresponding  tones.     (Landois  and  Stirling.) 

of  a  violin  depends  upon  age,  the  peculiarities  of  the  grain  of  its  wood, 
and  the  shape,  size,  and  connections  of  the  various  parts  of  the  instru- 
ment. 

The  muscular  adjustments  necessary  in  producing  all  the  wonderful 
variations  in  tone  and  in  quality  of  which  the  human  voice  is  capable  are 
inconceivably  delicate.  Not  only  the  muscles  of  the 
larynx,  but  those  of  the  various  accessory  vocal  organs, 
the  tongue,  the  lips,  the  palate,  and  the  pharynx,  and  the  respiratory 
muscles,  contribute  their  share  in  the  process.  All  of  these  muscles  are 
under  the  most  careful  nervous  control,  and,  as  we  shall  see  later,  a  par- 
ticular area  of  the  brain  has  as  its  special  duty  the  management  of  the 
vocal  organs.  The  training  of  the  voice  is  a  training  of  these  nervous 
and  muscular  mechanisms.  That  which  determines  whether  a  voice  shall 
be  called  soprano,  contralto,  tenor,  or  bass,  is  partly  the  natural  length  of 
the  vocal  cords  and  partly  the  general  nature  of  the  vocal  mechanism. 
The  average  range  of  the  individual  voice  is  two  to  two  and  a  half  oc- 
taves ;  the  relative  pitches  of  the  four  kinds  of  voice  are  shown  in  the 
accompanying  table  (Fig.  21). 

In  singing  a  scale  we  are  conscious  of  the  necessity  at  certain  notes  of 
rearranging  our  vocal  organs  if  we  wish  to  produce  well-rounded  tones 
and  prevent  the  voice  from  breaking.  Such  adjust- 
ments take  place  at  different  notes  for  different  indi- 
viduals.    The  compass  that  is  possible  for  each  adjustment  constitutes 


Vocal  Registers. 


128       PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN  HEALTH. 

the  so-called  "  register."  Vocal  teachers  detect  several  registers,  but 
physiologists  recognise  commonly  two — the  chest  register  or  voice,  and  the 
head  register  or  voice.  The  former,  employed  for  low  notes,  is  charac- 
terized by  richness  and  fulness  of  tone ;  the  latter,  employed  for  high 
notes,  is  thinner.  The  difference  in  the  mechanism  of  the  two  is  not 
fully  understood,  and  is  perhaps  not  the  same  for  all  individuals.  The 
appearance  of  the  vocal  cords  during  the  production  of  voice,  as  shown 
by  the  laryngoscope  (a  small  mirror  placed  in  the  back  of  the  mouth  and 
reflecting  a  bright  light  down  into  the  larynx),  is  presented  in  the  accom- 

B 

■J..  A 


Fig.  22. — Inteeiob  of  larynx,  as  seen  by  laryngoscope,  during  production  of  (A)  chest- 
voice (Mandl  and  Griitzner),  (B)  head-voice  (Mills). 

The  glottis  is  represented  as  a  black  longitudinal  slit  in  the  middle  of  the  figures,  long  in  A,  short 
in  B ;  the  vocal  cords  are  shaded  in  A,  white  in  B ;  the  curved  body  at  the  upper  part  of  the 
figures  is  the  epiglottis ;  the  rounded  elevations  at  the  lower  part  of  the  figures  are  cartilages. 

panying  figure  (Fig.  22).  The  mechanism  of  the  falsetto  voice  in  man 
is  also  in  dispute.  The  breaking  of  the  voice  in  boys  at  puberty  is  caused 
by  the  rapid  growth  of  the  larynx  and  the  constant  congested  condition 
of  the  vocal  cords. 

Speech  is  voice  modified  by  changes  in  the  accessory  vocal  organs, 
especially  the  resonance  cavities,  the  pharynx,  the  nasal  cavities,  and  the 
mouth.  The  sounds  of  speech  are  classified  into  vowels  and  consonants. 
All  vowels  have  the  same  laryngeal  sound  as  their  basis ;  but  for  each 
vowel,  by  changes  in  the  shape  of  the  resonance  cavities,  different  over- 
tones are  added  to  the  fundamental  laryngeal  tone,  hence 
the  difference  in  the  sounds.  Consonants  are  noises 
produced  mainly  in  the  mouth  by  modifications  of  the  outgoing  current 
of  air.  Some  are  and  some  are  not  accompanied  by  vocal  sounds.  In 
gutturals  (K,  G)  the  modification  is  produced  by  the  soft  palate  and  the 
root  of  the  tongue ;  in  dentals  (T,  D,  S,  L,  Z,  1ST,  R)  by  the  tip  of  the 
tongue  near  the  teeth ;  in  labials  (P,  B,  F,  V,  M)  by  the  lips.  The  details 
of  the  mechanisms  of  the  consonants  must  be  omitted. 


FUNCTIONS  OF  AMOEBOID  AND  CILIATED  CELLS.         129 

Section  III. 
NON-MUSCULAR  MOTOR  MECHANISMS. 

Muscle  is  not  the  only  motor  tissue  that  is  found  in  the  human  body. 
There  are  two  other  varieties  of  contractile  cells  that  in  a  very  unostenta- 
tious way  perform  mechanical  work  and  are  indispensable  to  the  body's 
welfare.     These  are  amoeboid  cells  and  ciliated  cells. 

Amoeboid  cells  comprise  the  colourless  corpuscles  of  blood  and  of 

lymph  (Fig.  4,  G),  and  are  so  called  because  they  resemble  and  are  capable 

of  moving  about  from  place  to  place  like  the  simple  one- 
Amoeboid  Cells.  ,,,         •       t       A         7  t>   /•  ^  ■<        -ii 

celled  animal,  Amceba.     Keterence   has   already   been 

made  to  their  function. 

Ciliated  cells  are  epithelial  cells,  and  are  fixed  in  position  with  one 

end  exposed  to  the  cavity  which  they  line.     This  uncovered  end  bears  a 

tuft  of  minute,   delicate,  hairlike  filaments   (the   cilia) 
Ciliated  Cells.         ,,  ■,•.,-,  .,      m.       -,    -i-in       -^      •        -,.,.      -, 

that  project  into  the  cavity  (rig.  1,  L).    During  life  the 

cilia  are  in  constant,  rapid,  wavelike  motion,  sweeping  along  whatever 
substances  come  in  contact  with  them.  They  are  especially  useful  in  car- 
rying from  the  lungs  toward  the  mouth  and  the  nose  mucus,  and  with  it 
inhaled  particles  of  dust.  They  line  not  only  the  bronchial  tubes,  the 
trachea,  the  larynx,  and  the  nasal  cavities,  but  the  ducts  of  certain  other 
organs,  and,  being  in  incessant  action  throughout  the  lifetime,  they  ac- 
complish a  large  amount  of  labour. 


CHAPTER  III. 

THE  NERVOUS  SYSTEM. 

It    would    be    a  sorry    community    of    people  wherein    every    indi- 
vidual worked  for  himself  alone,  regardless  of  the  wants  and  the  welfare 

of  others,  and  wherein  there  existed  between  individuals 
Nervous  System  ,  .  ~  •   i        i  •   i  • 

.    r,         ,         and  between  professions  no  social  and  no  commercial  in- 
tra tfenerai.  L 

tercourse.  A  continuance  of  such  a  state  of  things 
would  be  impossible  unless  the  community  were  composed  of  few  indi- 
viduals and  such  as  were  content  to  remain  low  in  the  scale  of  civilization. 
The  same  principles  apply  to  a  community  of  protoplasmic  cells  and 
organs ;  if  there  be  no  intercellular  and  no  interorganic  comity  and  ex- 
change there  is  no  rising  in  the  scale  of  organisms.  We  have  seen  that 
in  the  human  body  the  mutual  interactions  of  the  various  parts  are  exces- 
sively complex,  and  this  complexity  is  carried  so  far  that  no  part  is  able 
to  live  when  separated  from  the  body.  In  both  the  community  of  people 
11 


130       PHYSIOLOGY:    THE  VITAL  PROCESSES  IN  HEALTH. 


and  the  organic  community  an  agent  is  needed  to  control  the  relations  of 
individuals.     Such  an  agent  exists  in  the  system  of  government  of  the 

one  and  the  nervous  system  of  the  other. 
In  every  body  of  men  that  has  taken  rank 
above  the  lowest  a  government  exists, 
while  in  every  protoplasmic  animal  or- 
ganism above  the  simplest  there  is  a 
nervous  system.  The  subordinate  posi- 
tion in  the  organic  world  that  is  ac- 
corded to  plants  is  due  more  than  all  else 
to  their  lack  of  nervous  organs.  The 
nervous  system  is  at  once  the  servant  and 
the  master  of  all  the  other  systems ;  it 
responds  to  the  needs  of  one  by  control- 
ling the  work  of  another ;  thus  it  co- 
ordinates and  harmonizes,  and  makes  one 
of  many.  But  it  attends  not  only  to 
internal  affairs  :  it  keeps  the  organism 
apprised  of  what  goes  on  without,  and 
thus  enables  the  body  to  adapt  itself  to 
its  environment.  It  is,  finally,  the  medi- 
um of  all  mental  life.  To  accomplish  all 
this  it  must  of  necessity  be  complicated 
both  in  its  anatomy  and  in  its  mode  of 
working.  No  system  in  the  body  is  more 
complicated.  None  is  more  difficult  to 
investigate  and  to  understand.  The  nerv- 
ous system  of  man  comprises  the  central 
nervous  system,  consisting  of  the  brain 
and  the  spinal  cord,  and  the  peripheral 
nervous  system,  consisting  of  the  nerves 
and  the  ganglia.  A  portion  of  the  pe- 
ripheral system  is  known  as  the  sympa- 
thetic system,  though  this  can  not  be  re- 
garded as  physiologically  independent  of 
the  rest.  To  this  enumeration  must  be 
added  the  organs  of  the  special  senses; 
these  are  so  unique  as  to  justify  treat- 
ment in  a  separate  chapter.  The  differ- 
ent parts  of  the  nervous  system  have 
very  different  structures  and  functions, 
such  that  the  whole  may  be  regarded  as  a  union  of  numerous  complex 
organs ;  but  notwithstanding  the  complexity,  the  elements  of  structure 


Fig.  23. — Diagram  of  a  typical  necron. 
6,  cell  body  ;  d,  dendrites,  or  protoplasmic 

processes ;  a,  axis-cylinder  process,  or 

nerve  fibre. 


STRUCTURE  AND  FUNCTION  OF  THE  NERVOUS  SYSTEM.    131 


and  of  function  are  fundamentally  the  same  throughout  all  parts.  "Within 
the  past  ten  years  remarkable  advances  in  our  knowledge  of  nervous 
structures  have  been  made. 

The  elements  of  nervous  structure  are  the  nerve  cells,  or  neurons,  as 
they  are  now  coming  to  be  called.     Neurons  vary  in  shape,  but  each  con- 
sists of  a  cell  body  and  processes  extending  from  it 

Neii'ZftruLre.    ^  23>     The  cel1  ho^  consists  of  Protoplasm  and  a 
large  nucleus.    The  processes  may  be  of  two  kinds,  called 

dendrites,  or  protoj)lasmic  processes,  and  axis-cylinder  processes.  The 
dendrites  are  much-branched,  short  filaments.  The  axis-cylinder  process, 
usually  one  in  number  for  each  cell,  is  the  most  highly  specialized  part  of 
the  neuron.  It  has  few  branches,  and  may  be  very  long  (three  to  four  feet). 
Near  its  end  it  splits  into  numerous  fine  filaments  that  terminate  in  the 
vicinity  of  the  cells  supplied  by  the  neuron,  whether  they  be  muscle  cells, 
gland  cells,  sense  cells,  or  other  neurons.  The  brain  and  the  spinal  cord  are 
masses  of  neurons  bound  together  by  connective  tissue  and  richly  permeated 
by  blood-vessels.  It  was  formerly  supposed  that  the  processes  are  joined 
together  into  an  inextricable  network,  and  that  nervous  impulses,  in  pass- 
ing from  one  part  of  the  nervous  system  to  another,  traverse  this  maze. 
But  recent  discoveries 
have  made  it  reason- 
ably certain  that  there 
is  no  network  what- 
ever ;  that,  on  the  other 
hand,  every  neuron  is 
independent  of  every 
other,  and  that  a  nerv- 
ous impulse  passes 
from  one  to  another 
through  contact,  and 
not  through  continuity 
of  their  respective  pro- 
cesses. In  some  parts 
of  the  brain  and  the 
spinal  cord  cell  bodies 
and  dendrites  predomi- 
nate and  constitute  the 
gray  matter ;  in  other 
parts  axis-cylinder  pro- 
cesses only  exist,  constituting  the  white  matter.  Nervous  ganglia  are 
masses  of  cell  bodies.  Nerves  are  bundles  of  axis-cylinder  processes  ar- 
ranged parallel  to  each  other,  each  process  being  ensheathed  usually  in  a 
fatty  covering  called  the  medullary  sheath.      The  axis-cylinder  process 


Fig.  24. — Psychic  brain  cells  in  different  stages  of  evo- 
lution. 

A,  frog ;  B,  newt :  C,  mouse ;  D,  man.  Series  a  to  e  shows  the 
stages  that  a  single  brain  cell  of  a  higher  vertebrate  passes 
through  in  its  growth.     (Baker,  after  BamGn  y  Cajal.) 


132      PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN  HEALTH. 


Elements  of 
Nervous  Function. 


and  its  protecting  sheath  form  a  nerve  fibre,  and  every  nerve  is  composed 
of  numerous  nerve  fibres.  In  the  embryo  nerve  cells  arise  as  compact 
bodies.  The  processes  appear  as  outgrowths  from  them,  and  continue  to 
grow  in  length  and  complexity  during  embryonic  life  and  adolescence. 
It  is  a  significant  fact  that  the  neurons  are  more  complex  the  higher  they 
are  in  the  animal  scale  (Fig.  24). 

The  elements  of  nervous  function  comprise  the  functions  of  the  body 
of  the  neuron  and  those  of  the  processes.  The  cell  body  is  the  central 
organ  of  nervous  energy.  It  receives,  originates,  and 
gives  out  nervous  impulses.  In  most  neurons  the  activ- 
ity of  the  cell  body  is  wholly  unconscious,  but  in  those 
existing  in  the  superficial  layers,  the  cortex  of  the  cerebrum,  mental  phe- 
nomena accompany  the  nervous  actions,  hence  such  neurons  are  called 
psychic.     The  processes  of  the  nerve  cell  are  specialized  to  conduct  nerve 

impulses,  the  den- 
drites conducting 
probably  toward  the 
cell  body,  the  axis- 
cylinder  process  in 
some  cells  away  from, 
in  others  toward,  the 
cell    body.       Nerve 


REFLEX    CENTRE  - 


AUTOMATIC  CENTRE — 


o- 


sensorv  cell  and  cells  are  usually  said 

AFFERENT  NERVE 

to    act    either    auto- 


MOTOR  CELL  AND 
EFFERENT  NERVE 


'  MOTOR  CELL  WITH 
EFFERENT  NERVE 


matically  or  reflexly. 
An  automatic  action 
is  one  in  which  the 
impulse  originates  in 
the  cell  body  as  the 
result  of  chemical  or 
other  changes,  and 
passes  thence  along 
the  axis-cylinder  pro- 
cess to  the  end  organ 
(Fig.  25).  The  re- 
spiratory centre  is  said  to  act  automatically.  The  psychic  cells  are  called 
automatic.  It  is  a  question  whether  automatism  in  this  sense  is  at  all  a 
common  phenomenon.  A  reflex  action  is  one  in  which  the  nervous  im- 
pulse originates  outside  of  the  nerve  cell,  passes  to  the  latter,  is  there 
elaborated,  and  then  passes  on  as  before  to  the  end  organ  (Fig.  25).  A 
typical  example  of  a  reflex  action  is  that  of  winking  :  a  foreign  body 
touches  the  eyelashes  or  the  eyeball ;  this  causes  nervous  impulses  to  go 
to  the  nerve  cells  that  control  the  muscles  of  the  lids ;  return  impidses 


Fig.  25.— Diagram  to  illustrate  nervous  mechanism  in  (1)  auto- 
matic ACTION,  (2)  REFLEX  ACTION',  (3)  PASSAGE  OF  SENSORY  IM- 
PULSE UPWARD  AND  OF  MOTOR  IMPULSE  DOWNWARD  WITHIN  CEN- 
TRAL  NERVOUS   SYSTEM.      (Mills.) 


THE  NATURE  OF  REFLEX  ACTIONS. 


133 


Nerve  Centres  mid 
Nerve  Conductors. 


come  back  to  the  muscles,  and  the  lids  close.  Keflex  actions  are  invol- 
untary. The  greater  part  of  the  body's  actions  are  reflex ;  not  only  the 
unconscious  movements  that  we  are  making  constantly  by  means  of  our 
skeletal  muscles,  but  also  the  muscular  movements  of  the  viscera  and 
secretion  in  glands.  The  term  "reflex  arc"  signifies  the  anatomical 
apparatus  required  for  a  reflex  action.  It  consists  of  (1)  a  sensory  end 
organ  ;  (2)  an  afferent  nerve  fibre ;  (3)  its  associated  nerve-cell  body ;  (4) 
a  second  nerve-cell  body  in  functional  connection  with  (3),  and  giving  rise 
to  (5)  an  efferent  nerve  fibre ;  (6)  a  motor  end  organ,  usually  a  muscle. 

Thus  we  see  that,  physiologically,  the 
nervous  system  consists  of  innumerable  nerve 
centres  and  nerve  conduct- 
ors. The  bodies  of  the 
nerve  cells  are  the  centres  ; 
the  processes,  especial  }j  the  axis-cylinder 
processes,  are  the  conductors.  Considered 
en  masse  and  roughly,  the  gray  matter  of 
the  central  nervous  system  and  the  ganglia 
outside  of  it  have  the  functions  of  centres ; 
the  white  matter  of  the  central  nervous  sys- 
tem and  the  nerves  are  conducting  in  func- 
tion. No  fibre  conducts  in  more  than  one 
direction.  The  nerve  fibres  outside  of  the 
brain  and  the  spinal  cord  may  be  divided 
into    two   great   classes,   according   as    they 

conduct  impulses  toward  the  central  nervous  system  or  away  from  it ; 
accordingly,  they  are  known  either  as  centripetal  or  afferent,  or  as 
centrifugal  or  efferent  fibres.  Most  nerves  are  composed  of  both  kinds. 
In  the  case  of  the  spinal  nerves  a  separation  of  the  two  kinds  takes 
place  at  the  junction  of  the  nerve  and  the  spinal  cord,  such  that  the 
posterior  or  dorsal  root  consists  of  afferent,  the  anterior  or  ventral  root 
of  efferent  fibres.  An  afferent  impulse,  upon  arriving  at  its  centre, 
may  simply  give  rise  at  once  to  an  unconscious  efferent  impulse,  pro- 
ducing a  reflex  action,  or,  with  or  without  doing  this,  it  may  pass  up- 
ward to  the  brain  and  give  rise  to  a  sensation  (Figs.  25  and  26).  Corre- 
spondingly, an  efferent  impulse  may  either  arise  in  a  lower  reflex  centre, 
as  the  direct  result  of  an  afferent  impulse,  or  it  may  arise  high  up,  even  in 
the  psychic  part  of  the  brain,  and  pass  downward  and  outward,  giving  rise 
to  a  voluntary  act.  Hence,  in  harmony  with  the  classification  of  periph- 
eral fibres  into  afferent  and  efferent,  there  occurs  within  the  brain  and  the 
cord  a  distinction  between  such  fibres  as  conduct  upward  toward  the  cere- 
brum and  such  as  conduct  downward  from  the  cerebrum.  The  former 
are  really  paths   for  the  continuation  of  the  afferent  impulses  coming 


Fig.  26. — Diagram  intended  to 
show  the  relations  of  the 
brain,  the  spinal  cokd,  and 
the  peripheral  organs. 
sensory  end  organ  ;  C,  spinal 
cord  ;  M,  motor  end  organ  (mus- 
cle); H,  hemisphere  of  brain. 
(James.) 


s, 


134      PHYSIOLOGY:    THE  VITAL  PROCESSES  IN  HEALTH. 

from  tlie  outside  to  the  psychic  cells,  where  the  impulses  may  give  rise  to 
sensations;  hence  afferent  nerve  fibres  and  those  that  conduct  upward 
within  the  brain  and  the  spinal  cord  are  often  called  sensory.     On  the 

other  hand,  the  downward  impulses  within  the  central 
Sensory  and  Motor.  .       .       ,  .     ,  .        ,. 

nervous  system  are  destined  in  large  part  for  the  mus- 
cles, and  pass  to  them  along  the  efferent  nerve  fibres,  hence  such  conduct- 
ing paths  are  called  motor.  The  same  terms  apply  to  the  cell  bodies 
which  the  fibres  join.  The  distinction  between  sensory  cells  and  motor 
cells,  sensory  fibres  and  motor  fibres,  sensory  centres  and  motor  centres, 
and  sensation  and  motion  as  nervous  functions,  is  one  of  the  most  funda- 
mental distinctions  in  the  physiology  of  the  nervous  system.  It  is  a 
curious  and  as  yet  not  explained  fact  that  the  sensory  parts  of  the  brain 
and  the  spinal  cord  lie,  in  general,  dorsal  or  posterior  to  the  motor 
parts. 

We  have  now  presented  the  elements  of  nervous  action.     The  central 
nervous  system  is  a  collection  of  central  stations  for  the  receipt,  trans- 
formation, and  transmission  of  nervous  energy.     Each 

,T  . ,  . .         of  these  stations  has  its  own  specific  function,  but  they 

Jyervous  Action.  m  r  '  .      ■' 

are  joined  with  each  other  in  the  most  intricate  manner, 
and  they  are  continually  modifying  each  other's  work.  In  ascending 
from  the  lower  to  the  higher  parts  of  the  nervous  system  there  is  a  pro- 
gressive correlation  of  functions  and  a  supervision  of  the  lower  by  the 
higher  centres.  At  the  top  in  the  cortex  of  the  cerebrum  lie  the  psychic 
cells,  which  are  the  physical  media  of  mental  life,  the  seat  of  the  sensa- 
tions and  the  place  of  origin  of  voluntary  acts,  and  which  are  able  to  con- 
trol the  acts  of  all  the  lower  centres  (Fig.  26).  To  this  elaborate  mechan- 
ism stream  constantly  through  sensory  nerves  impulses  from  all  parts  of 
the  body  and  from  the  organs  of  the  special  senses,  giving  information  re- 
garding the  condition  and  the  needs  of  the  various  organs  and  tissues  and 
the  occurrences  of  the  outside  world.  The  ultimate  possible  goal  of  these 
impulses  is  the  cortex  of  the  cerebrum,  there  to  give  rise  to  sensations. 
But  very  few  of  the  impulses  reach  this  goal.  Those  to  which  the  atten- 
tion is  directed,  those  which  are  unusually  or  excessively  strong,  or  which 
for  other  reasons  require  consideration  by  the  mind,  pass  to  the  cortex. 
The  great  majority,  however,  are  dealt  with  by  the  lower  centres. 
Wherever  the  impulses  terminate  they  act  upon  sensory  centres  ;  these  in 
turn  stimulate  motor  centres ;  and,  largely  as  the  result  of  the  incoming 
stream,  there  is  as  constant  an  outgoing  stream  of  motor  and  other  im- 
pulses through  efferent  nerves  to  the  tissues  and  the  organs.  These  out- 
going nervous  impulses  regulate  the  actions  of  the  various  parts  and  of 
the  body  as  a  whole.  Let  us  now  consider  the  parts  of  the  central  nervous 
system  more  in  detail. 

As  we  ascend  through  the  series  of  vertebrate  animals  in  the  order, 


THE   BRAIN  AND  SPINAL  CORD. 


135 


fish,  reptile,  bird,  mammal,  man,  we  find  that  there  is  a  progressive  in- 
crease in  the  weight  of  the  brain  as  compared  with  the 
Comparative  Im-  .   1  ,      *  , -,      ■,      ,  „,  .     .       ,  .  ,  , 

t         f  D     ■       weight  of  the  body.      I  Ins  is  shown  in  round  numbers 
portance  of  Brain.  &  J 

in  the  accompanying  table  (Waller) : 


Weight  of  brain. 

Weight  of  body. 

Pish 

Bird 

1 

1 
1 
1 

1 
1 

5,000 

1,500 

220 

180 

120 

Man. . .           

50 

Moreover,  there  is  a  progressive  increase  in  the  size  of  the  brain  as 
compared  with  the  spinal  cord  ;  within  the  brain  there  is  a  progressive 
increase  in  the  size  and  complexity  of  the  higher  parts  as  compared  with 
the  lower ;  and,  lastly,  there  is  a  progressive  increase  in  the  size  and 
relative  importance  of  the  cerebral  cortex.  This  last  fact  is  to  be  corre- 
lated with  the  gradual  evolution  and  perfecting  of  mind,  while  the  facts 
together  mean  that  in  general  there  is  a  progressive  subordination  of  lower 
to  higher  centres.  Accordingly  we  find,  in  ascending  the  series,  a  gradual 
limiting  of  the  work  of  the  lower  parts. 

A  fish  or  a  frog  will  continue  to  live  for  days  after  the  brain  is  wholly 

destroyed  if  the  spinal  cord  be  left  intact.     This  is  impossible  in  the  case 

of  man.     The  human  spinal  cord  is  pre-eminently  the 
Spinal  Cord.  ,  £        ,,  a  ,.  .  ,  .  , 

central  nervous  organ  for  the  reflex  actions  in  which 

the  spinal  nerves  take  part,  hence  for  the  actions  of  the  limbs  and  the 
trunk  (Fig.  27).  The  "  tone  "  of  the  voluntary  muscles  depends  upon  it. 
It  contains  respiratory,  vaso-motor,  and  other  centres  which  are  accessory 
to  more  powerful  ones  in  the  medulla  oblongata.  Obviously  it  is  also  the 
path  of  conduction  of  impulses  between  the  spinal  nerves  and  the  brain. 
The  majority  of  these  impulses  ascend  and  descend  upon  the  side  of  the 
cord  from  which  their  nerves  arise.  A  few  cross  over  to  the  opposite  side. 
The  medulla  oblongata  and  the  rest  of  the  brain  stem  contain  the 
reflex  centres  of  the  cranial  nerves  and  various  other  automatic  or  reflex 
centres ;  they  regulate  the  movements  of  the  eyes,  the 
face,  the  tongue,  the  alimentary  canal,  the  heart,  res- 
piration, the  arteries,  the  larynx  in  speaking,  the  pharynx  and  oesophagus 
in  swallowing ;  they  also  control  the  secretion  of  saliva  and  other  digest- 
ive fluids.  The  brain  stem  serves  also,  like  the  spinal  cord,  as  a  path  of 
upward  and  downward  impulses.  It  is  a  curious  fact  that  both  sensory 
and  motor  impulses  here  cross  over  from  one  side  of  the  brain  to  the 
other ;  hence  the  left  side  of  the  cerebrum  deals  with  the  right  half  of  the 
body,  and  vice  versa. 


Brain  Stem. 


136       PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN   HEALTH. 


The  function  of  the  cerebellum  is  commonly  believed  to  be  that  of 

harmonizing  or  co-ordinating  the  actions  of  the  muscles.     Injury  to  this 

part  of  the 
Cerebellum.  ,      .  ,, 

brain  results 

in  irregularity  and  uncertain- 
ty of  bodily  movements.  But 
experimental  evidence  has  not 
made  it  clear  exactly  how  the 
organ  acts. 

Exact  knowledge  regard- 
ing the  functions  of  the  optic 
thalamus  and  the  corpus  stri- 
atum is  quite  wanting. 

The  cortex  of  the  cere- 
brum is  spoken  of  as  the 
"seat"      or 


Cortex  of  Cerebrum. 


the "organ' 


of  consciousness  and  of  intel- 
ligence ;  its  cells  form  the 
physical  basis  of  mental  phe- 
nomena. When  a  man  thinks, 
his  cortical  cells  act;  and  if 
the  latter  be  destroyed,  men- 
tal phenomena  seem  to  cease 
so  far  as  that  individual  is 
concerned.  We  can  only 
speculate  as  to  the  kind  of 
relation  that  exists  between 
the  cerebral  and  the  mental 
processes  ;  so  far  as  we  know, 
the  latter  are  always  accom- 
panied by  the  former.  As 
experimental  physiologists  we 
search  for  the  cerebral  pro- 
cesses, and  we  find  that  apparently  they  do  not  differ  from  those  that 
take  place  in  any  mass  of  nerve  cells  whose  activity  is  unaccompanied  by 
consciousness.  Structurally  the  cortex  is  unique,  being  characterized  by 
the  presence  of  the  large,  much-branched  pyramidal  cells  that  occur 
nowhere  else  in  the  organism  (Fig.  28) ;  in  details  it  differs  in  different 
parts.  Formerly  it  was  believed  that  the  cortex  acts  as  a  whole,  but 
modern  research  has  shown  the  untenability  of  this  view,  which,  indeed, 
seems  now  opposed  to  common  sense. 

In  1870  two  German  physiologists,  Fritsch  and  Hitzig,  found  that  if 


Fig.  27. — Diagram  to  illustrate  reflex  actions. 
A,  skin;  F,  F',  muscle  fibres;  large  column  at  left 
represents  a  piece  of  the  spinal  cord  with  gray  mat- 
ter within  and  white  matter  without;  B,  dorsal 
nerve  roots :  C,  ventral  nerve  roots ;  N  i,  N'  1,  N  2, 
N'  2,  N  3,  N'  3,  neurons.  A  simple  reflex  path  com- 
prises N 1  and  N  2,  which  include  origin  of  axis- 
cylinder  process  in  end  bulb  (a)  or  between  cells 
(A),  afferent  fibre  (<j,  e)  belonging  to  cell  body  (d), 
branches  (/)  giving  off  "  collateral "  branches  (g) 
to  terminate  about  N  2 ;  cell  body  (h),  efferent  fibre 
(i)  terminating  upon  muscle  fibre  (F).  A  compound 
reflex  path  comprises  a  similar  collecting  neuron, 
N'  1,  one  or  more  correlating  neurons,  N '  2,  and  dis- 
tributing neurons,  N'  3.    (Baker.) 


FUNCTIONAL  DIFFERENTIATION  OF  CORTICAL  AREAS.     137 


the  brain  of  a  dog  were  exposed  and  the  cortex  stimulated  in  different 
places  by  electric  shocks,  contractions  of  the  animal's  muscles  accom- 
panied   each    stimulation, 

and  the  movements  varied     \.    ,    <k  f,i/   ,      .  .'.,.■»  .'■•/      ,';,.;, ;\i;: 
according  to  the  particu- 
lar corti- 
Localization  of       ^    apea 
Functions  %n 
the  Cortex.  that  was 

touched. 
Since  then  investigation 
of  the  subject  by  various 
methods  upon  various  ani- 
mals and  man  has  been 
active  with  the  result  that 
now  localization  of  differ- 
ent functions  in  different 
parts  of  the  cortex  has 
become  an  accepted  fact, 
although  there  is  still 
considerable  difference  of 
opinion  regarding  the  ex- 
act functions  of  different 
areas.  The  accompanying 
figures  represent  the  opin- 
ions of  authorities  at  pres- 
ent, but  the  details  are  sub- 
ject to  change  by  future 
investigation  (Figs.  29 
and  30).  The  most  obvi- 
ous principle  regarding 
localization  is  that  the 
motor  area,  which  has  the 
function  of  controlling 
through  lower  centres  the 
actions  of  the  various  vol- 
untary muscles,  is  situated 
in  the  middle  part  of  the 
cortex,  extending  across 
the  top  of  the  brain ; 
while  the  sensory  area, 
which  deals  with  the  sen- 
sations arising  from  activ- 
ity of  the  organs  of  the 


cr£" 


— kS. 


Fig.    28. — Section    of    the    cerebral    cortex    of    yolng 

mouse,  showing  psychic  cells  and  their  branches. 
A-D,  various  layers  of  cells ;  i?,  white  matter;  <?,  deudrites 

or  protoplasmic   processes ;   c,  e,  axis-cylinder   processes ; 

b,  a,/,  g.  A,  different  types  of  cells.     (Baker,  after  Ramon 

y  Cajal.) 


138       PHYSIOLOGY:    THE  VITAL  PROCESSES  IN  HEALTH. 

special  senses  and  of  the  sensory  nerves,  lies  farther  back.     The  motor 
area  lies  in  general  on  either  side  of  the  fissure  of  Rolando,  and,  as  is 


£ 


<fl  o 


-3  e 


2  io 


■°.S 


-s 


1*3 

Woo 


I    b  oja- 


1     "       ri  H3 

rt  5S.S 
H  O  ^ 


seen  in  the  figures,  the  centres  for  the  actions  of  different  groups  of 
muscles  have  been  localized.     The  centre  marked  speech  is  the  motor 


THE  UNDERSTANDING  AND  THE  USE  OP  LANGUAGE.     139 


centre  for  the  muscles  of  the  organs  of  speech.     In  the  living  man  it 
exists   in  the  area  a   little    behind   and   above   the   left   temple.      The 


(5 


6 


& 

a 


?< 


whole  process  of  the  understanding  and  the  use  of  language  is  exces- 
sively complicated,  involving  many  parts  of  the  brain ;  a  portion  of 


140       PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN   HEALTH. 


the  nervous  mechanism  is  represented  in  Fig.  31.  Visual  impulses  («") 
from  the  printed  words  pass  to  the  centre  of  sight  (V) ;  auditory  im- 
pulses (a)  from  the  spoken  words  pass 
to  the  centre  of  hearing  (A) ;  these 
centres  are  connected  by  "  association  " 
fibres  with  each  other  and  with  the  motor 
centres  for  the  arm,  as  used  in  writ- 
ing (W),  and  for  the  vocal  organs  (E) ; 
when  these  muscles  are  employed,  motor 
impulses  (to',  to)  pass  to  them,  while  sen- 
sory impulses  («',  s)  inform  the  centres  of 
what  is  going  on.  The  various  centres 
do  not  appear  to  be  sharply  marked  off, 
but  rather  to  overlap  adjacent  areas. 
Most  of  them  exist  upon  both  sides  of 
the  brain,  and,  as  has  been  stated,  each 
half  of  the  brain  controls  the  opposite 
side  of  the  body.  The  right-handedness 
of  most  persons  is  due  to  the  superiority 
and  greater  refinement  of  the  left  hemi- 
sphere of  the  brain  over  the  right,  and 
the  reverse  is  true  of  left-handed  indi- 
viduals. Most  persons  are  "  left-brained 
speakers,"  since  the  speech  centre  is  de- 
veloped usually  upon  that  side  only. 

The  functions  of  the  frontal  lobes  of 
the  cerebrum  are  quite  unknown.  The 
higher  psychical,"  in  Fig.  29,  should  be  accepted  with  caution. 
We  have  not  space  here  to  go  into  a  discussion  of  them, 
nor  of  the  physical  bases  of  the  various  psychical  phe- 
nomena— the  will,  the  feelings,  attention,  judgment,  memory,  etc.  All 
such  subjects  are  still  in  a  very  hypothetical  stage. 

The  modern  doctrine  of  localization  recalls  the  older  school  of  phre- 
nology and  proves  how  utterly  unscientific  and  unfounded  were  the 
phrenological  conceptions.  Where  the  phrenologist 
located  "hope"  the  scientific  physiologist  finds  the 
nervous  mechanism  of  locomotion  ;  "  inhabitiveness  "  and  "  self-esteem  " 
marked  the  centres  now  known  to  be  concerned  in  vision.  A  bump  upon 
the  skull  does  not  necessarily  imply  a  bump  upon  the  brain ;  and  the 
laboratory  work  of  Fritsch  and  Iiitzig  relegated  phrenology  to  the  in- 
evitable oblivion  of  empiricism. 

In  discussing  the  nervous  system  we  have  tacitly  assumed  that  the 
function  of  any  one  part  is  simply  to  stimulate  some  other  part  to  act. 


Fig.  31. — Diagram  intended  to  show 
the  nervous  mechanism  employed 
in  the  understanding  and  use  of 
language  (for  description,  see  text). 
(James,  after  Eoss.) 


term 


Psychical  Processes. 


Phrenology. 


SUPPEESSION  OR  INHIBITION  OF  ACTION. 


141 


Suppression  or  inhibition  of  action  is,  however,  widespread,  and  of  almost 

equal  importance.     The  best-known  example  of  inhibition  is  that  of  the 

vagus  nerve  slowing  or  stopping  the  beat  of  the  heart. 
Inhibition.  _,  *"  .     ,.    .  ,      ,    ,  ,  „,  ,  . 

.hi very   individual    knows    how  often   in   Ins  conscious 

life  he  is  compelled  to  say  "  ISTo  "  to  his  impulsive  self,  and  an  objective 
study  of  his  own  nervous  processes,  if  such  were  possible,  would  show 
him  that  one  of  the  frequent  tasks  of  his  nerve  cells  is  to  prevent  or  put 
a  stop  to  the  actions  of  other  nerve  cells.  Probably  any  act  of  the  indi- 
vidual beyond  the  very  simplest  is  the  result  of  a  combination  of  motor 
and  inhibitory  nervous  influences.  Consider  the  nervous  processes  as 
portrayed  in  the  accompanying  figure  (Fig.  32).  The  baby  sees  the 
candle  flame  for  the  first  time  and  instinctively  tries  to  grasp  it ;  the 
nerve  paths  of  this  simple  reflex  action  are  shown  in  1,  1,  1,  1,  from  the 
eye  to  the  visual  centre,  thence  to  the  motor  centre  and  to  the  muscles 
that  extend  the  hand.  The  finger  is  burned,  and  a  second  reflex  results 
in  the  withdrawal  of  the  hand ;  2,  2,  2,  2,  marks  the  reflex  arc,  from  the 
sensory  nerve  endings  in  the  skin  to  the  sensory  centre,  thence  to  the 
motor  centre  and  to  the  muscles  that  with- 
draw the  hand.  But  the  cerebral  centres 
are  apprised  of  the  occurrences,  and  the 
upgoing  impulses  arouse  perceptions  of  (si) 
the  image  of  the  flame,  (ml)  the  action  of 
extension,  («2)  the  pain,  and  (m2)  the  action 
of  withdrawal.  The  groups  of  cells  mediat- 
ing the  perceptions  are  "associated"  by 
fibres,  and  retain  in  their  physical  structure 
the  "  memory  "  of  the  events.  A  few  days 
later  seeing  the  candle  arouses  again  the 
unconscious  impulse  to  seize  it.  But  the 
cortical  cells  are  now  on  the  alert,  inhibi- 
tory impulses  are  shot  down  to  the  motor 
centres,  and  the  hand  is  withheld. 

The  nature  of  nervous  impulses  or  nervous  energy  is  not  known. 

When  nerve  centres  act,  their  protoplasm  appears  to  undergo  katabolic 

changes,  to  produce  heat,  to  become  fatigued,  and  thus 

„  „  ■  to  resemble  other  protoplasm  in  its  general  metabolic 

JS'ervous  Jinergy.  i  J  ° 

phenomena.  Nerve  fibres,  however,  are  peculiar  in 
that  no  evidences  of  katabolic  changes  are  present  in  them  during  even 
long-continued  passing  of  nerve  impulses.  The  rate  of  transmission  of 
impulses  is  not  difficult  to  measure,  and  in  man  averages  about  one  hun- 
dred feet  in  the  second.  A  feeble  electric  current  always  accompanies  the 
impulse,  but  the  slow  rate  and  other  considerations  seem  to  forbid  the 
assumption  that  the  impulse  itself  is  electrical. 


Fig.  32. — Diagram  intended  to 
show  the  paths  of  nervous  im- 
PULSES (for  description,  see  text). 
(James.) 


142      PHYSIOLOGY  :    THE  VITAL  FKOOESSES  IN   HEALTH. 

During  the  past  few  years  physiological  psychologists  have  devoted 
much  attention  to  the  measurement  of  the  time  of  cerebral  processes — 
the  time  it  takes  to  think.  The  basis  of  their  investiga- 
tions is  the  determination  of  the  simple  reaction  time, 
which  is  the  time  that  elapses  between  the  giving  of  a  stimulus — say 
sending  an  electric  shock  to  the  skin  or  light  to  the  e}-e — and  a  signal 
made  by  the  person  to  indicate  that  the  stimulus  is  felt.  The  average  re- 
action time — which,  it  is  needless  to  say,  varies  greatly  in  different  indi- 
viduals— is  between  one  tenth  and  two  tenths  of  a  second.  It  is  shortest 
for  touch,  longest  for  sight,  and  for  hearing  it  is  intermediate  between 
the  other  two  (touch  014  seconds,  hearing  0'16  seconds,  sight  0T8  sec- 
onds). If  the  time  required  by  the  end  organs  and  the  nerves  be  sub- 
tracted from  the  whole  interval  between  stimulation  and  reaction,  there 
is  left  the  time  occupied  by  the  brain  itself  in  recognising  the  sensation 
and  willing  the  motor  response ;  this  may  be  placed,  in  round  numbers, 
at  one  tenth  of  a  second.  Practice  and  attention  shorten  the  reaction 
time ;  fatigue  and  complication  of  the  cerebral  process  (such  as  would  be 
caused  by  offering  the  subject  a  choice  between  two  kinds  of  stimulation) 
lengthen  it. 


CHAPTER   IV. 

SENSATION. 

All  parts  of  the  body  are  supplied  with  sensory  nerve  fibres,  and  ac- 
cordingly from  all  parts  of  the  body  impulses  may  go  to  the  central  ner- 
vous system  and  there  give  rise  to  sensations.    Sensations 

Sensations  in  ,        .n     n  . ,,  -,  •   ^        r\  i 

a        ,  are  classified  as  either  general  or  special,     (general  sen- 

sations comprise  those  by  which  we  recognise  vaguely 
the  existence  and  condition  of  the  various  parts  of  our  bodies ;  bodily 
comfort  and  discomfort,  fatigue  and  pain,  are  general  sensations;  all 
parts  of  the  body  seem  to  possess  general  sensibility.  Special  sensations 
comprise  those  that  possess  a  more  specific  distinguishing  quality  than 
general  sensations ;  they  are  mediated  by  specialized  organs,  which  are 
confined  to  certain  specific  parts  of  the  body.  Formerly  it  was  custom- 
ary to  recognise  five  classes  of  special  sensations — namely,  those  of 
sight,  hearing,  taste,  smell,  and  touch.  Investigation  has  now  shown  that 
two  others  must  be  added  to  the  list — namely,  sensations  of  temperature 
and  muscular  sensations.  The  anatomical  apparatus  of  each  of  the  seven 
senses  consists  of  (1)  delicate  end  organs,  which  are  adapted  in  each  case 
to  a  special  method  of  stimulation  ;  (2)  special  afferent  nerve  fibres ;  (3) 


STEUCTUEE  AND  FUNCTION   OF  THE  EYE. 


143 


special  portions  of  the  central  nervous  system  which  are  the  seat  of  the 
sensations.  Of  all  the  senses,  sight  has  the  most  highly  specialized  or- 
gans, and  may  profitably  be  considered  first. 


Section  I. 
SIGHT. 


The  sense  of  sight  arouses  in  us  ideas  of  form,  size,  distance,  light, 
shade,  and  colour.  The  end  organs  of  sight  are  the  eyes,  which  are 
adapted  to  stimulation  by  the  vibrations  of  the  ether  that  we  call  light ; 
the  afferent  nerves  are  the  optic  nerves ;  and  the  seat  of  visual  sensation 
is  the  cortex  of  the  occipital  lobes  of  the  cerebrum. 


SUPERIOR  RECTUS 


CHOROID 


:  OPTIC  NERVE- 


INFERIOR  RECTUS 


Fig.  33. — Section  of  the  eyeball  (Flint/ 


Considered  as  a  piece  of  physical  apparatus,  the  eyeball  is  a  camera 
obscura,  of  which  the  best-known  example  is  the  photographic  camera — 
that  is,  it  consists  of  a  chamber  with  blackened  walls. 
omyofthe  Eye      ^e  ^ac^  wa^  no^s  a  sensitive  plate,  the  retina,  upon 
which  an  image  of  tbe  object  is  thrown ;  the  front  wall 
is  pierced  by  a  hole,  the  pupil,  about  which  is  an  adjustable  diaphragm, 
the  iris  ;  and  a  system  of  transparent  refractive  bodies,  or  lenses,  com- 
prising the  aqueous  humour,  the  crystalline  lens,  and  the  vitreous  hu- 
mour, bring  the  rays  of  light  to  a  focus  upon  the  retina  (Fig.  33).     The 


144       PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN  HEALTH. 


end  to  be  readied  is  the  stimulation  of  the  nervous  apparatus  in  the  retina, 
and  accordingly  the  retina  is  regarded  as  the  essential  part  of  the  eye,  all 
other  parts  being  accessory. 

In  its  origin  the  retina  is  a  part  of  the  brain,  which  has  grown  out 
along  the  route  of  the  optic  nerve  and  has  come  to  be  located  in  the  eye- 
ball. It  consists  of  nerve  cells  of  different  shapes,  ar- 
ranged in  layers,  as  shown  in  the  accompanying  figures 
(Figs.  34  and  35),  and  bound  together  by  supporting  tissue.  The  sig- 
nificance of  the  various  parts  is  not  understood,  but  the  terminal  end 


Retina. 


Choroid  coat. 


A 


V 

Path  Path 

of  of 

rays  nervous 

of  impulses. 

light. 


10 Layer     of     pig- 
ment cells. 

g  Layer    of    rods 

and  cones. 

3  External     limit- 
ing membrane. 

7   Outer       nuclear 

layer. 

g    Outer  molecular 

layer. 

er  Inner       nuclear 

u  layer. 


A   Inner  molecular 

layer. 


Layer  of    nerve 
ceils. 


.Layer  of    nerve 
fibres. 

j  .  ..Internal  limiting 
membrane. 


Vitreous  humour. 
Fig.  34 — Section  of  the  retina  (diagrammatic  ).     (Schultze.) 


organs  of  the  nervous  mechanism  are  the  rods  and  the  cones,  which, 
through  forming  the  outer  layer  of  the  retina,  seem  to  be  the  parts  that 
are  sensitive  to  light,  and  upon  which  the  image  is  focused.  They  con- 
sist of  highly  specialized  protoplasm  ;  their  names  and  the  figures  indicate 
their  shape.     It  is  estimated  that  there  are  at  least  three  million  cones 


RETINAL  IMAGES  AND  AFTER-IMAGES. 


145 


After-images. 


and  many  more  rods  in  each  retina.  The  difference  in  function  of  the 
rods  and  the  cones  is  not  known.  The  light  probably  causes  chemical 
changes  in  them,  and  thus  originates  nervous  impulses ;  the  latter  traverse 
the  various  retinal  layers  of  nerve  cells  and  their  processes,  reach  the 
optic  nerve,  and  go  thence  to  the 
brain.  The  retina  may  be  stimu- 
lated mechanically  by  pressure  upon 
the  eyeball.  Thus,  rubbing  the  eyes, 
as  is  often  done  upon  awaking  from 
sleep,  produces  sensations  of  light 
in  the  form  of  points,  spots,  or  cir- 
cles. "  Seeing  stars,"  as  the  result 
of  a  blow,  is  due  to  mechanical  stimu- 
lation of  the  retina. 

The  effect  upon  the  retina  lasts 
frecmently  for  a  considerable  time 
after  the  eye  is  turned  away  from 
the  object.  For 
example,  a  single 
look  at  the  sun  will  enable  us  to  see 
suns  for  several  seconds  afterward. 
Such  an  after-effect  is  termed  an 
after-image.  Sometimes,  and  espe- 
cially at  first,  it  is  of  the  same  colour 
as  the  object,  but  later  it  appears  in 
the  opposite  or  complementary  col- 
our ;  in  the  former  case  it  is  called 
positive,  in  the  latter  negative.  Thus 
the  after-image  of  a  white  object  may  be  at  first  white,  but  later  it  is 
gray  or  black  ;  of  a  red  object,  perhaps  momentarily  red  but  soon  green. 
Positive  after-images  are  due  to  a  continuation  of  the  nervous  excitation 
after  the  cause  of  it  is  removed  ;  negative  after-images  result  from  fatigue 
of  the  nervous  mechanism  for  the  colour  that  is  looked  at,  hence  the 
predominance  of  the  complementary  colour. 

Tbe  place  of  entrance  of  the  optic  nerve  into  the  retina,  being  devoid 
of  rods  and  cones,  and,  in  fact,  of  all  nervous  organs  except  nerve  fibres, 
is  insensitive  to  light,  and  is  called  the-  blind  spot.  Its 
blindness  may  be  proved  by  the  following  simple  ex- 
periment :  If  the  left  eye  be  closed  and  the  circular  disk  in  Fig.  36  be 
looked  at  with  the  right  eye,  it  will  be  found  that  the  cross  is  also  visible, 
except  when  the  book  is  held  at  a  distance  of  from  nine  to  twelve  inches 
from  the  face ;  at  that  distance  the  image  of  the  cross  falls  upon  the  end 
of  the  optic  nerve  and  is  not  perceived.  In  ordinary  life  each  eye  cor- 
12 


Fig.  35. — Section  of  the  retina  (diagram- 
matic), INTENDED  TO  ILLUSTRATE  THE  RE- 
CENT DISCOVERIES  AS  TO  THE  STRUCTURE 
AND   RELATIONS   OF   THE   VARIOUS   LAYERS. 

A,  layer  of  rods  (a)  and  cones  (b) ;  _B,  outer 
nuclear  layer;  c,  nucleus  of  cone;  d,  nu- 
cleus of  rod ;  C,  outer  molecular  layer ;  D, 
inner  nuclear  layer ;  e,  rod-bipolar  cell ;  /, 
cone-bipolar  cell ;  i?,  inner  molecular  layer ; 
F,  layer  of  nerve  cells;  g,  7i,i,j,  k,  nerve 
cells  with  processes  branching  at  different 
levels ;  £r,  layer  of  nerve  fibres ;  £,  non- 
nervous  supporting  cell  (fibre  of  Muller). 
(Ramon  y  Cajal). 


Blind  Spot. 


146       PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN  HEALTH. 

rects  the  invisible  spot  in  the  field  of  the  other,  and  hence  we  do  not 
appreciate  the  two  gaps  in  our  combined  field  of  vision. 


Fig.  36. — Diagram  for  demonstrating  existence  of  blind  spot. 


Formation  of 
Retinal  Image. 


The  formation  of  the  retinal  image  is  a  purely  physical  matter,  and 
follows  the  laws  of  refraction  of  light.  The  rays  of  light  from  each 
portion  of  the  object  looked  at  are  bent  out  of  their 
course  by  the  three  refractive  bodies,  but  chiefly  by  the 
crystalline  lens,  and  are  brought  to  a  focus  upon  the 
layer  of  rods  and  cones.  Thus  a  small  inverted  image  of  the  object 
appears  upon  the  retina  exactly  as  upon  the  ground  glass  of  the  photo- 
graphic camera  (Fig.  37).  As  in  all  optical  instruments,  focusing  is 
necessary  in  order  that  the  picture  may  be  sharp  and  distinct.  By  the 
curvature  of  the  lens  and  the  length  of  the  eyeball  the  eye  is  focused 
normally  for  objects  situated  at  a  great  distance  from  the  observer.  For 
nearer  objects  focusing  might  be  brought  about  theoretically  either  by 
making  the  lens  more  convex  or  by  increasing  the  length  of  the  eyeball. 
In  the  camera  the  latter  method  is  employed  by  drawing  back  the  plate 
of  ground  glass.  In  the  eye  the  retina  can  not  be  moved,  and  hence  the 
curvature  of  the  lens  is  altered  in  the  following 
manner :  The  lens  is  elastic,  and  normally  is 
kept  slightly  flattened  by  the  tension  of  sur- 
rounding parts.  When  vision 
Accommodation.  ..        ,     ,    ,  .  .  ,, 

is  directed  to  near  objects  the 

ciliary  muscle,  which  lies  just  outside  the  edge 
of  the  lens,  contracts  and  pulls  forward  the 
choroid  coat  together  with  the  suspensory  liga- 
ment of  the  lens.  Tension  on  the  lens  being 
thus  diminished,  the  latter  bulges  forward  and 
becomes  more  convex  (Fig.  38).  Sharpness  of  the  image  is  also  assisted 
by  shutting  out  the  rays  that  would  come  through  the  more  external 
parts  of  the  lens.  This  is  accomplished  by  the  muscular  diaphragm,  the 
iris,  which,  in  proportion  to  the  nearness  of  the  object,  contracts  and 


Fig.  37. — Diagram  illustrat- 
ing REFRACTION  OF  RAYS  OF 
LIGHT  AND  FORMATION  OF  AN 
INVERTED    IMAGE    BY   A    LENS. 

A  B,  object;  L,  lens;  i  a,  im- 
age.   (Martin.) 


NEAR-SIGHT,   FAR-SIGHT,   ASTIGMATISM,  AND  OLD-SIGHT.    147 

diminishes  the  size  of  the  pupil.  Both  the  iris  and  the  ciliary  muscle  are 
uncfer  nervous  control,  and  their  actions  are  delicately  harmonized  reflexly 
through  the  brain,  the  optic  nerve  being  the  afferent  nerve. 

Considered  as  an  optical  instrument,  the  human  eye,  although  appar- 
ently so  exactly  adapted  to  its  uses,  is  by  no  means  perfect.     Indeed,  the 

distinguished  German  physicist  and  physiologist,  Helm- 
Visual  Apparatus.    lloHz'  whlle  admiring  its  surpassing  fitness,  once  wrote 

of  its  shortcomings  :  "  If  an  optician  wanted  to  sell  me 
an  instrument  which  had  all  these  defects,  I  should  think  myself  quite 
justified  in  blaming  his  carelessness  in  the  strongest  terms,  and  giving 
him  back  his  instrument."  The  eyeball  may  be  too  long  (near-sight)  or 
too  short  (far-sight);  in  both  cases  exact  focusing  for  a  wide  range  of 
vision  is  difficult,  and,  unless  the  eye  is  assisted  by  spectacle  lenses,  a 
blurred  image  falls  upon  the  retina.  The  curvature  of  the  front  surface 
of  the  eyeball,  the  cornea,  may  be  irregular,  a  very  common  defect  known 
as  astigmatism,  which  manifests  itself  also  by  a  blurred  image.     The 


Fig.  38. — Diagram  illustrating  the  mechanism  or  accommodation  of  the  ete. 
In  if  the  lens  is  accommodated  for  near  objects;  in  F,  for  distant  objects.    (Fick.) 


crystalline  lens,  like  all  lenses,  produces  some  spherical  and  some  chro- 
matic aberration.  Floating  within  the  vitreous  humour  of  all  eyes  are 
peculiar  opaque  particles,  cells  of  irregular  shape  and  filaments,  that 
appear  as  mysterious  moving  particles  in  the  field  of  vision ;  they  are 
most  evident  when  one  gazes  at  a  white  wall  or  at  the  sky ;  they  are 
entirely  harmless.  A  considerable  number  of  individuals  suffer  from 
colour  blindness,  which  is  apparently  due  to  a  lack  of  certain  colour-per- 
ceiving elements  in  the  retina  and  will  be  discussed  hereafter.  Lastly 
may  be  mentioned  the  trouble  with  vision  that  appears  in  most  persons 
shortly  after  middle  life  (old-sight) ;  it  may  be  due  to  a  weakening  of  the 
ciliary  muscle  or  to  a  loss  of  elasticity  of  the  crystalline  lens. 

The  question  as  to  how  we  see  colour  has  been  and  still  is  the  cause 
of  much  speculation,  experimentation,  and  theorizing.  Objectively  one 
colour  differs  from  another  only  in  the  length  of  the  waves  and  the  rapid- 


148       PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN   HEALTH. 

ity  of  vibration  of  the  ether ;  and  white  light  is  the  resultant  of  a  host 

of  these  different  waves  taking  place  simultaneously.     By  means  of  the 

spectroscope  white  light  may  be  analyzed  into  seven 
Colour  Vision  . 

colour   groups,    called    "  primary    colours "    viz. :    red, 

orange,  yellow,  green,  blue,  indigo,  and  violet ;  and,  since  given  these 
colours  all  colours  whatsoever  may  be  obtained  by  proper  mixing,  it  fol- 
lows that,  if  we  can  explain  the  perception  of  the  primary  colours,  we 
can  explain  all  colour  vision.  But  the  matter  is  simpler  than  this,  for 
experiment  shows  that  all  colour  sensations  may  be  produced  by  proper 
mixing  of  three  colours  only  instead  of  seven — e.  g.,  red,  green,  and  violet. 
This  fact  is  the  basis  of  the  common  theory  of  colour  vision,  the  Young- 
Helmholtz  theory,  which  supposes  the  existence  of  three  primary  colour 
sensations,  such  as  red,  green,  and  violet,  all  other  colour  sensations  being 
combinations  of  these.  Perhaps  these  correspond  to  three  kinds  of 
material  elements  in  the  rods  and  cones  or  in  other  parts  of  the  visual 
apparatus,  each  element  being  capable  of  stimulation  by  all  colours  of 
light,  but  more  especially  by  one  colour.  Thus  white  light  stimulates  all 
three  alike,  red  light  the  red-perceiving  elements  more  strongly,  and  so 
on.  Colour-blindness  is  due  to  a  lack  of  at  least  one  of  the  three  kinds 
of  elements — a  red  ribbon  appears  like  a  green  ribbon  because  the 
red-perceiving  elements  are  wanting.  This  theory  accounts  for  many 
of  the  facts  of  colour  vision,  but  is  not  entirely  sufficient.  Several  other 
theories  have  been  suggested  and  are  being  actively  tested  and  dis- 
cussed. 

With  some  of  the  lower  animals,  such  as  the  rabbit,  every  object  is 
seen  with  one  eye  only ;  each  eye  has  its  own  field  of  vision.  Man,  how- 
,  „.  .  ever,  is  a  binocular  animal,  since  under  ordinary  cir- 
cumstances  he  directs  his  two  eyes  toward  the  object 
looked  at.  As  a  matter  of  fact,  only  the  middle  part  of  his  field  of  vision 
is  binocular,  since  the  right  eye  alone  sees  to  the  extreme  right,  the  left 
eye  alone  to  the  extreme  left ;  the  amount  of  overlapping  of  the  right 
and  the  left  fields  in  front  depends  upon  the  shape  and  the  prominence 
of  the  nose.  Slightly  different  pictures  of  the  object  looked  at  fall  upon 
corresponding  points  in  his  two  retinas,  but  out  of  the  two  images  he 
receives  a  sensation  of  one  object.  Binocular  vision  is  of  the  greatest 
value,  since  by  it  we  are  able  to  form  much  more  exact  ideas  of  distance, 
size,  and  form  than  by  one  eye  alone.  Binocular  vision  would  be  nearly 
valueless,  however,  were  it  not  supplemented  by  the  muscles  of  the  .eye- 
balls and  their  exact  co-ordination  by  the  central  nervous  system.  These 
muscles  rival  those  of  the  larynx  in  delicacy  of  action,  and  enable  any 
point  within  the  range  of  the  two  eyes  to  be  turned  to  instantly.  The 
newly  born  babe  often  possesses  the  primitive  power  of  moving  the  eyes 
independently  of  each  other,  a  power  which  adults  rarely  retain. 


SECEETION  OF  THE  LACHRYMAL  GLAND.  149 

The  eyelids  exist  for  the  protection  of  the  eyeballs.  Winking,  by 
sweeping  across  the  ball  and  through  the  lachrymal  canals  into  the  nose 
the  secretion  of  the  lachrymal  gland,  serves  to  keep  the 
surface  of  the  cornea  moist,  and  to  wash  away  foreign 
particles  that  might  be  injurious  to  it.  When  the  secretion  becomes  ex- 
cessive, it  overflows  in  the  form  of  tears.  Lachrymal  glands  arose  in  the 
course  of  evolution,  when  aquatic  animals  gave  rise  to  those  leading  a 
terrestrial  life.  Hence  fishes,  whose  eyes  are  bathed  constantly  by  the 
surrounding  water,  neither  require  nor  possess  them,  while  they  exist  in 
all  animals  above  fishes.  The  power  of  weeping,  although  it  has  been 
observed  in  elephants,  some  monkeys,  and  in  a  few  other  animals,  is  con- 
fined chiefly  to  man.  Usually,  but  not  always,  it  signifies  grief  or  bodily 
suffering.  Darwin  believes  that  it  arose  incidentally  as  the  result  of  a 
chain  of  events  somewhat  as  follows :  The  suffering  child,  like  the  young 
of  other  animals,  cries  out,  partly  for  aid  from  its  parents,  partly  to 
relieve  itself  by  activity ;  in  this  act  the  blood-vessels  of  the  eyes  are 
gorged  with  blood,  and  to  prevent  injury  therefrom  the  muscles  about 
the  eyes  are  contracted  strongly  (see  Fig.  20,  B) ;  the  muscular  contrac- 
tion causes  in  some  reflex  way  not  wholly  understood  a  flow  of  tears  from 
the  gland ;  in  the  adult  all  the  events  in  the  chain  are  not  necessary,  and 
grief  leads  directly  to  weeping. 


Section   II. 

HEARING. 

The  ear  is  the  most  complicated  of  all  known  sense  organs.      Its 

essential  part  comprises  the  nerve-endings  in  the  membranous  labyrinth 

of  the  internal  ear ;  the  accessory  parts  consist  of  the 
Physiological  Anat-  ,      .    ,,       ..  -,  ,,'  .,j,  ,,, 

tt-u    t?  rest  of  the  internal  ear,  the  middle  ear,  and  the  exter- 

omy  of  the  Ear.  '  ' 

nal  ear.  In  so  far  as  the  ear  is  used  for  hearing,  it  is 
adapted  to  receive  the  vibrations  of  air  that  we  call  sound  and  to  transmit 
them  to  the  nerve  terminations.  The  diagram  (Fig.  39)  shows  the  rela- 
tions of  the  various  parts  of  the  auditory  organ.  The  pinna,  or  external 
part  that  we  commonly  call  the  ear,  is  omitted  from  the  figure,  but  the 
passage  (K  M.,  external  auditory  meatus)  is  shown,  and  the  arrow  indi- 
cates the  path  of  the  waves  of  sound.  The  waves  cause  the  tympanic 
membrane  (Ty.  M.)  to  vibrate,  and  this  produces  a  gross  movement  of 
the  delicate  ear  bones  {Mall.,  Inc.,  Stp.)  that  stretch  across  the  air- 
space of  the  middle  ear  (Ty.).  The  middle  ear  is  seen  to  be  really  a 
drum  with  one  drumhead  (Ty.  M.)  upon  one  side,  two  drumheads 
(F.  o.,  F.  r.)  upon  the  opposite  side,  and  a  passage  (Eu.)  to  the  pharynx, 
and  thus  to  the  outside  air,  for  the  purpose,  as  in  all  drums,  of  equalizing 


150       PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN  HEALTH. 


the  pressure  of  air  within  and  without.     Through  the  membrane  of  the 
fenestra  ovalis  (F.  o.)  the  liquid  perilymph  within  the  bony  labyrinth 

{Sea.  V.,  Sea.  T)  of  the 
internal  ear  is  set  into  vi- 
bration. The  vibrations 
pass  through  the  walls  of 
the  membranous  laby- 
rinth, affect  the  liquid 
endolymph,  and  stimulate 
the  nerve-endings  (Sea. 
M.).  A  more  detailed 
but  still  diagrammatic 
figure  of  the  membranous 
labyrinth  is  shown  in 
Fig.  40.  It  is  there  seen 
that  the  labyrinth  con- 
sists of  three  distinctive 
parts,  viz. :  two  central 
irregular-shaped  cavities 
(utricle  and  saccule)  in- 
directly connected  ;  three 
semicircular  canals  join- 
ing the  utricle,  each  with 
an  enlargement,  the  am- 
pulla ;  and  the  canal  of 
the  cochlea  (Coch.),  join- 
ing the  saccule.  Each  of 
the  three  distinctive  parts 
contains  in  its  walls  char- 
acteristic nerve  terminations ;  and,  as  the  figure  shows,  there  are  in  real- 
ity six  end-organs  within  the  ear.  Of  these  it  is  probable  that  the  ner- 
vous organ  in  the  cochlea  alone,  the  organ  of  Corti,  is  auditory  in 
function. 

The  organ  of  Corti  (Fig.  41)  is  a  complex  mechanism,  resting  upon 
one  wall  of  the  cochlear  canal,  the  basilar  membrane,  and  extending 
from  the  base  to  the  apex  of  the  cochlea.  It  consists  of 
a  succession  of  curious  rodlike  structures,  the  rods  of 
Corti,  beside  which  are  columnar  cells,  tipped  with  hairs  and  surrounded 
by  the  terminations  of  the  nerve  fibres ;  the  hairs  pass  through  a  perfo- 
rated membrane  (reticulate  membrane),  and  over  all  is  a  curtain,  the  tec- 
torial membrane.  The  hair-cells  are  the  proper  end-organs  ;  they  corre- 
spond in  function  to  the  rods  and  cones  of  the  retina ;  through  them  the 
nerve  fibres  are  stimulated.     But  the  method  of  stimulation  has  not  yet 


Fig.  39. — Diagram  of  ear. 

The  external  ear  comprises  the  external  projecting  part,  or 
pinna  (omitted  from  figure),  and  the  external  auditory 
meatus  (E.  M.) ;  the  arrow  indicates  the  direction  of  the 
waves  of  sound.  The  middle  ear  or  tympanum  {Ty.) 
comprises  the  tympanic  membrane  (  Ty.  J/1),  the  fenestra 
ovalis  (F.o.).  the  fenestra  rotunda  (F.r.),  the  three  bones, 
malleus  {Mall.),  incus  (Inc.),  and  stapes  (Stp.),  and  the 
Eustachian  tube  (Eu.) ;  the  middle  ear  is  filled  with  air, 
and  connects  with  the  pharynx  through  Eu.  The  inter- 
nal ear,  or  labyrinth,  comprises  the  osseous  labyrinth  and 
the  membranous  labyrinth.  Both  are  highly  diagram- 
matic in  figure,  the  former  being  reduced  to  scala  tympani 
(Sea.  T.)  and  scala  vestibuli  (Sea.  V.)  of  cochlea,  and  one 
semicircular  canal,  the  latter  being  reduced  to  scala 
media  (Sea.  M.)  of  cochlea,  and  one  semicircular  canal 
(M.L.);  the  osseous  labyrinth  is  filled  with  perilymph, 
the  membranous  labyrinth  with  endolymph.  All  bony 
parts  are  shaded  with  oblique  lines.     (Huxley.) 


Organ  of  Corti. 


THE  EAR  AND  A  THEORY  OF  HEARING. 


151 


A.N. 


been  satisfactorily  determined.  It  will  be  remembered  tbat  the  organ  of 
Corti  is  bathed  with  endolymph  and  the  bony  labyrinth  outside  the  coch- 
lear canal  is  filled  with  perilymph,  hence  the  vibrations  caused  by  sound 
may  be  transmitted  readily  to 
the  organ  of  Corti.  It  was 
formerly  thought  that  the  rods 
of  Corti  first  received  these  vi- 
brations, and  that  from  them 
the  nerves  were  stimulated. 
But  it  seems  more  reasonable 
to  believe  that  the  important 
part  of  the  organ  is  the  basilar 
membrane,  which  consists  of 
parallel  fibres  extending  from 
within  outward,  and  which  may 
act  somewhat  after  the  manner 
of  the  wire  board  of  a  piano. 

In  discussing  voice  we  spoke 
of  pitch,  quality,  and  loudness 
as  three  characteristics,  the  pro- 
duction of  which  was  to  be  ex- 
plained. A  theory  of  hearing 
must  explain  the  recognition  of  these  three  characteristics.  According 
to  the  theory  now  usually  adopted,  the  recognition  of  pitch  depends  upon 


Fig.  40.- 


P.S.C. 


Coch. 
Diagram  of  membranous  labyrinth  and 
distribution  of  auditory  nerve. 
U,  utricle ;  A.  S.  ft,  B.  S.  ft,  and  P.  S.  ft,  anterior,  ex- 
ternal, and  posterior  semicircular  canals ;  A.  V., 
aquseductus  vestibuli ;  S.,  saccule  ;  ft,  canalis  re- 
uniens ;  Coch.,  canal  of  cochlea,  or  scala  media ; 
A.  N.,  auditory  nerve,  dividing  into  six  branches 
that  end  respectively  in  the  three  crista}  acusticce 
of  the  ampulla?  of  the  semicircular  canals,  the  two 
maculm  acusticm  of  the  utricle  and  the  saccule, 
and  the  organ  of  Corti  in  the  cochlea.     (Huxley.) 


Fig.  41. — Diagram  of  cross-section  of  canal   of  cochlea,  showing  organ  of  Corti  resting 

upon  basilar  membrane. 
z,  inner,  and  y,  outer  rod  of  Corti ;  i,  inner,  and  p,  outer  hair  cells ;  N,  nerve,  and  n,  nerve  fibrils 

Eassing  to  hair  cells  ;  o,  reticulate  membrane :  Mb.  Corti,  membrane  of  Corti,  or  tectorial  mem- 
rane ;  d,  K,  o,  //,  ft,  epithelial  cells.    (Landois.) 


152      PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN  HEALTH. 

the  particular  part  of  the  basilar  membrane  that  is  put  into  vibration. 
The  fibres  of  which  the  membrane  is  composed,  about  twenty-four  thou- 
sand in  number,  become  increasingly  longer  from  the 
Recognition  of       bage  to  the        x  of  the  coehiea.     it  ;s  conceivable  that, 
Fitch,  Quality,  and      .  .  -1  ... 

Loudness.  ins^  as  ls  tne  case  ln  tne  Pianoj  the  longer  fibres  vibrate 

to  the  lower  notes  only,  the  shorter  fibres  to  the  higher 
notes  only ;  and  with  each  note  the  hair-cells  that  rest  upon  that  portion 
of  the  membrane  and  their  contiguous  nerve  fibres  are  stimulated. 
Quality  of  sound  depends  upon  the  number  and  prominence  of  the  over- 
tones that  accompany  the  fundamental  tone.  When  a  piano-key  is 
struck  and  a  single  wire  is  put  into  vibration,  other  wires  vibrate  in 
unison  and  produce  overtones.  So  in  the  basilar  membrane  of  the 
ear,  when  one  part  vibrates  other  parts  vibrate,  and  nervous  impulses  are 
produced  corresponding  to  the  fundamental  tone  and  the  overtones  of 
the  sound  that  comes  to  the  ear.  Recognition  of  loudness  depends  upon 
the  extent  of  vibration  of  the  basilar  membrane.  Therefore,  accord- 
ing to  the  theory  of  the  all-importance  of  the  basilar  membrane  in  hear- 
ing, a  sound  after  coming  to  the  ear  and  reaching  the  membranous 
labyrinth  is  analyzed  into  its  constituent  vibrations.  Nervous  impulses 
corresponding  to  the  various  vibrations  are  produced  in  the  fibres  of  the 
auditory  nerve,  are  transmitted  to  the  brain,  and  give  rise  to  sensations. 
These  various  sensations  are  combined  and  elaborated  into  a  percep- 
tion of  the  sound.  It  should  be  said  that  this  theory  is  not  wholly  satis- 
factory. 

The   semicircular   canals  and   their  nervous  end-organs,   the   cristas 

acusticce,  seem  to  have  nothing  whatever  to  do  with  hearing.     They  are 

sense-organs    of  bodily  equilibrium ;    thev  enable   the 

S\pY)l'}  CI  7*/")/ ffll* 

c      ,  individual  to  recognise  the  turning  of  the  head  and  of 

the  body  out  of  one  position  into  another.  They  are 
especially  prominent  in  fishes  and  in  birds — animals  that  spend  much  of 
their  time  in  diving  and  turning  in  fluid  media,  the  water  and  the  air 
respectively.  In  each  ear  the  planes  of  the  three  canals  are  nearly  at 
right  angles  to  each  other,  so  that  they  represent  the  three  planes  in 
space.  Each  canal  is  hollow  and  filled  with  endolymph,  and  its  crista 
acustica,  which  lies  in  the  enlarged  ampulla,  seems  specially  adapted  to 
stimulation  by  movement  of  the  contained  liquid  (Fig.  42).  The  cells 
about  which  the  nerve-fibres  terminate  are  tipped  with  long  hairs  that 
project  into  and  float  in  the  endolymph.  Any  curved  movement  of  the 
head  in  or  approximately  in  the  plane  of  a  canal  may  readily  cause  a 
bending  of  the  hairs  and  a  stimulation  of  the  nerves.  If  a  movement  of 
the  head  takes  place  in  any  other  plane,  two  or  more  canals  may  be 
stimulated  at  once,  and  by  means  of  the  six  canals  of  the  two  ears  acting 
in  various  combinations  curved  movements  in  all  possible  planes  in  space 


SENSE-ORGANS  OP  BODILY  EQUILIBRIUM. 


153 


Saccule  and  Utricle. 


may  be  appreciated.  Roughly  speaking,  the  canals  act  as  spirit  levels, 
and  the  delicacy  of  their  action  is  realized  when  we  consider  with  what 
accuracy  we  can  detect  the  slightest  turning  of  the  body  out  of  equi- 
librium. 

Whether  the  saccule  and  the  utricle  are  auditory  in  function  is  in 
great  doubt.  It  was  once  believed  that  their  nervous  terminations  appre- 
ciate noises  as  distinct  from  musical  sounds,  which 
mlate  the  cochlear  organs.  But  it  now  seems  prob- 
able that  the  cochlea  deals  with  both  kinds  of  sounds.  Structurally  the 
maoulce  acusticoe,  the  two  nervous  end-organs  upon  the  walls  of  the  sac- 
cule and  the  utricle,  are 
similar  to  the  cristas  of  the 
semicircular  canals ;  the 
hairs  are  shorter,  however, 
and  lying  upon  and  among 
them  is  a  mass  of  calca- 
reous crystals,  the  otoliths. 
Experiments  upon  fishes 
make  it  practically  certain 
that  these  organs  appreci- 
ate the  position  of  the 
head,  and  thus  of  the  body, 
in  space,  and  hence  are  or- 
gans of  equilibrium  of  the 
body  when  at  rest.  The 
mode  of  stimulation  is  be- 
lieved to  be  by  the  con- 
stant pressure  of  the  oto- 
liths upon  the  hair-cells,  and  the  production  thus  of  a  constant  nervous 
impulse  that  gives  the  individual  an  idea  of  his  position  ;  if  the  body  be 
thrown  out  of  the  normal  attitude,  the  pressure  of  the  otoliths  is  altered, 
and  thus  the  individual  recognises  his  new  position.  Experiments  upon 
fishes  indicate  also  that  the  saccule  and  the  utricle  appreciate  simple  pro- 
gressive bodily  movements  in  a  straight  line.  It  seems  not  unreason- 
able to  suppose  that  these  parts  may  have  in  man  the  same  functions 
as  in  the  lower  animals.  If  this  be  so,  the  semicircular  canals,  the  sac- 
cule, and  the  utricle  act  together  and  form  a  most  efficient  organ  of 
equilibrium  for  the  body  in  all  possible  movements  and  in  all  possible 
positions.  It  is  an  interesting  but  unsolved  problem  as  to  how  the  organ 
of  equilibrium  and  the  organ  of  hearing  became  associated  with  each 
other  in  the  ear. 


Fig.  42. — Diagram  of  longitudinal  section  of  ampulla 
of  semicircular  canal,  showing  crista  acustica  («*.) 

c,  opening  into  canal ;  ra,  opening  into  utricle;  e.  epithelial 
lining  of  ampulla;  w,  branch  of  auditory  nerve  supply- 
ing crista;  a. e.,  hair  cells;  a. h.,  hairs  projecting  into 
endolymph ;  c.  t.,  connective  tissue.     (Huxley.) 


154       PHYSIOLOGY:    THE  VITAL  PROCESSES  IN  HEALTH. 


Smell. 


Section   III. 

SMELL.     TASTE. 

Different  animals  vary  greatly  as  to  the  relative  importance  of  their 
various  sensations.  The  world  is  made  known  to  man,  for  example, 
chiefly  through  sight,  hearing,  and  touch.  The  dog's 
world  is  largely  a  world  of  odours,  and  in  him  the  sense 
of  smell,  comparatively  unimportant  in  man,  is  very  acute.  A  substance, 
in  order  to  be  smelled,  must  be  either  in  a  gaseous  or  in  a  very  finely 
divided  condition.  The  organs  of  smell  comprise  the  upper  portion  of  the 
two  cavities  of  the  nose ;  the  lower  portion  serves  as  a  passageway  for  air 
in  respiration  ;  hence  in  quiet  respiration  odours  are  not  usually  perceived  ; 
they  come  into  consciousness  only  when  they  are  very  strong  or  when 
by  sniffing  the  air  containing  them  is  forcibly  drawn  up  to  the  olfactory 
organs.  The  organs  are  very  simple,  consisting  solely  of  the  membrane 
lining  that  portion  of  the  nasal  cavities  (Figs.  43,  44).  The  membrane  is 
composed  of  columnar  epithelium  cells,  and  contains  glands  that,  together 
with  the  cells,  secrete  the  mucus  with  which  the  nose  is  always  moistened. 
Among  the  cells  are  scattered  long  delicate  ones  that  resemble  somewhat 
the  rods  of  the  retina,  and  from  which  nerve-fibres  go  directly  to  the 


Figs.  43  and  44. — Vertical  longitudinal  sections  of  the  cavity  of  the  nose. 
Fig.  4-3  represents  the  outer  wall  of  the  left  nasal  cavity ;  Fig.  44  the  inner  wall  of  the  right  nasal 
cavity.  ST,  MT,  IT,  superior,  middle,  and  inferior  turbinate  bones ;  Pa,  hard  palate  separating 
nasal  cavity  from  mouth  ;  Sp,  septum  or  partition  between  the  two  nasal  cavities;  7,-lles  with- 
in the  brain  cavity,  and  points  to  the  olfactory  nerve,  with  its  numerous  branches;  V,  branches 
of  the  trigeminus  or  fifth  nerve.     (Huxley.) 

brain  (Fig.  45).  These  are  the  olfactory  cells,  and  they  are  stimulated  by 
the  odoriferous  substance  coming  in  contact  with  their  exposed  ends. 
The  amount  of  substance  needed  to  cause  a  sensation  of  smell  is  incon- 


ORGANS  OF  SMELL  AND  TASTE. 


155 


Organs  of  Taste. 


ceivably  small ;  it  is  said  that  -000000005  of  a  milligramme  of  oil  of  pepper- 
mint suffices.  No  satisfactory  classification  of  odours  has  yet  been  made, 
analogous  to  that  of  colours  in  light  and  pitch  in  sound. 

The  sense  of  taste,  like  the  sense  of  smell,  is  relatively  unimportant  in 
man.  In  many  respects  it  is  not  unlike  the  sense  of  smell.  The  organs 
of  taste  comprise  certain  portions  of  the  lining  mem- 
brane of  the  mouth  cavity — viz.,  the  surface  of  the 
tongue,  the  soft  palate,  and  the  columns  upon  either  side  at  the  back  of 
the  mouth  (anterior  pillars  of  the  fauces).     Situated  in  the  covering  of 

these  parts  of  the  tongue,  upon  some  of  the 
papillae  that  roughen  its  surface,  are  small 
groups  of  sensory  cells,  called  taste -buds 
(Fig.  46).  The  gustatory  cells  composing 
them  are  not  unlike  the  olfactory  cells  in 
appearance.  They  are  sunk  a  little  below 
the  surface,  their  exposed  ends  being  reached 


3  2  1 

Fig.  45. — Nerve  and  end-organs 
of  sense  of  smell. 

1.  cells  from  nasal  cavity  of  frog; 
a,  epithelial  cell;  i,  olfactory 
cell ;  2,  branch  of  olfactory 
nerve  of  frog,  terminating  in 
fibrils;  3,  olfactory  cell  of 
6heep.    (Kolliker.) 


Fig.  46. — Two  taste-buds  from  the  rabbit's  tongue. 
(Magnified  450  diameters.)     (Engelmann.) 


Classes  of  Tastes. 


through  a  small  pore.  They  are  stimulated,  like  the  olfactory  cells,  by  a 
minute  quantity  of  the  substance  tha.t  is  to  be  tasted  coming  to  them  in 
solution.  It  is  possible  that  there  exist  other  organs  of  taste  whose 
function  is  as  yet  undiscovered. 

Like  colours  in  light  and  pitch  in  sound,  four  classes  of  tastes  are 
recognised,  viz.,  sweet,  sour,  bitter,  and  salty  ;  and  probably  all  tastes 
not  precisely  these  are  combinations  of  them.  Each  of 
these  classes  probably  has  its  own  end-organs  and  nerve- 
fibres.  The  front  part  of  the  tongue  tastes  sweet  and  sour  substances, 
the  back  part  bitter  substances.  Some  substances,  like  a  certain  com- 
pound of  saccharin,  when  placed  upon  the  tip  of  the  tongue  taste  sweet, 
upon  the  back  of  the  tongue  bitter,  because  they  are  able  to  stimulate 
both  kinds  of  end-organs.  Taste  is  assisted  greatly  by  smell,  the  two 
•senses  usually  working  intimately  together.  This  may  readily  be  proved 
by  the  easy  experiment  of  holding  the  nose,  closing  the  eyes,  and  then 


156       PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN  HEALTH. 

endeavouring  to  distinguish  by  taste  alone  between  a  bit  of  apple  and  a 
bit  of  onion,  or  between  a  particle  of  banana  and  a  strawberry. 


Section   IV. 

TOUCH.     TEMPERATURE.    MUSCULAR  SENSE.     GENERAL  SENSIBILITY. 

By  the  term  "  sense  of  touch  "  is  now  meant  the  sense  of  pressure. 
Its  organs  are  localized  in  the  skin,  and  consist  probably  of  various  known 
forms  of  touch  corpuscles.  These  are  minute  spherical 
or  ovoid  bodies,  consisting  of  cells  among  which  nerve- 
fibres  end,  the  structure  indicating  that  their  natural  method  of  stimula- 
tion is  by  pressure  upon  the  skin  overlying  them.  Recently,  physiologists 
have  succeeded  in  mapping  out  the  surface  of  the  skin  into  "  pressure 
points,"  which  recognise  the  touch  of  bodies  applied  to  the  skin,  and 
minute  areas  between  them,  which  are  wholly  devoid  of  the  sense  of 
touch.  The  "  pressure  points "  probably  overlie  the  organs  of  touch. 
The  forehead  and  the  back  of  the  hand  seem  to  be  among  the  most  sensi- 
tive parts  of  the  skin,  recognising  a  weight  of  -£-$  of  a  grain,  but  the  tips 
of  the  fingers  can  distinguish  more  exactly  two  objects  placed  near 
together. 

The  sense  of  temperature,  also  localized  in  the  skin,  has  only  recently 
been  recognised  as  a  sense  distinct  from  that  of  touch.  It  comprises,  in 
reality,  two  senses,  that  of  warmth  and  that  of  cold, 
which  are  now  believed  to  be  so  far  distinct  as  to  have 
separate  end-organs  and  separate  nerves.  Just  as  with  the  sense  of  pres- 
sure, so  with  temperature,  it  has  been  found  possible  to  map  out  the  sur- 
face of  the  skin  into  points  that  recognise  only  warmth  ("  warm  spots  ") 
and  points  that  recognise  only  cold  ("  cold  spots  ").  The  warm  spots  are 
distinct  from  the  cold  spots,  and  both  are  distinct  from  the  pressure 
points.  There  exist  in  the  skin  various  sensory  organs,  besides  the  touch 
corpuscles,  and  it  is  believed  that  some  of  these  mediate  sensations  of 
temperature,  biit  it  is  not  yet  decided  as  to  which  are  organs  of  warmth 
and  which  organs  of  cold.  The  face  is  the  region  of  the  skin  most  sensi- 
tive to  temperature. 

By  muscular  sensation  we  recognise  the  amount  of  contraction  that  a 
muscle  is  undergoing ;  and  from  such  data  we  form  judgments  of  the 

size,  form,  position,  and  weight  of  objects.     Since  mus- 
Muscular  Sense 

cular  contractions  occupy  so  large  a  field  in  our  daily 

life,  the  muscular  sense  is  important.     The  existence  of  a  muscular  sense 

can  be  indicated  by  a  simple  experiment :  If  an  object  be  held  in  the 

hand  and  be  moved  up  and  down,  we  have  a  distinct  consciousness  of  the 


GENEKAL  SENSIBILITY.  157 

tension  exerted  upon  the  muscles  of  the  arm  ;  from  this  muscular  feeling 
we  can  form  a  much  more  accurate  estimate  of  the  weight  of  the  object 
than  from  the  feeling  of  pressure  of  the  object  upon  the  skin  of  the 
hand  ;  the  feeling  of  pressure  distinct  from  that  of  muscular  contraction 
can  be  studied  by  laying  the  hand  flat  upon  the  table  and  placing  the  ob- 
ject in  the  palm.  What  the  sensory  end-organs  are  that  mediate  the 
muscular  sense  is  not  known  ;  nor  is  it  known  whether  they  are  localized 
in  the  muscles  themselves,  in  their  tendons,  or  in  the  joints.  There  is 
evidence  in  favour  of  each  of  these  three  localities. 

Although  sensations  of  touch,  temperature,  and  muscular  contraction 
are  regarded  as  distinct  and  as  mediated  by  three  kinds  of  end-organs, 

nerves,  and  brain-centres,  as  a  matter  of  fact  we  rarely 
Touch  Temperature,    ^  mQ  Qf  these  ^^  .      ^^      T}        aj.e  inextricably 

rarely  separated.      bound   together  in   ordinary  life.     Especially  are  our 

judgments   of  the  characteristics    of    objects   that  are 

touched  by  our  hands  formed  from  a  mixture  of  these  three  kinds  of 

sensations. 

General  sensibility  has  been  referred  to  as  the  property  by  which  we 

recognise,  in  a  vague  way,  the  existence  and  the  condition  of  the  various 

parts  of  our  bodies.     The  sensations  of  pain,  fatigue, 
Common  Sensation.    .  ,,  .  ...         ..   .  ,.  .  ,      ... 

hunger,  thirst,  shivering,  tickling,  nausea,  general  bodily 

comfort  and   discomfort,  have   all  been  referred  to  general  sensibility. 

Since  many  of  these  sensations  enter  largely  into  our  everyday  life,  it 

seems  strange  that  we  have  but  little  exact  anatomical  or  physiological 

knowledge  regarding  them.     Future  investigation  may  possibly  succeed 

in  lifting  some  of  them  out  of  the  ill-defined  group  of  common  sensations 

into  new  special  senses.     Pain  is  now  commonly  believed  to  be  due  to 

excessive  stimulation  of  nerves  of  general  sensation,  and  thus  to  have  a 

nervous  mechanism  distinct  from  those  of  the  seven  senses. 


CHAPTER   V. 

REPROD  UCTION. 

The  functions  so  far  considered  have  reference  to  the  daily  life,  well- 
being,  and  preservation  of  the  individual  man  or  woman.    But  the  human 

being  has.  in  common  with  other  organisms,   the    in- 

Beproduction  in         ,..'.,  ,.  ,  .   ,  ,. 

r         ,  stincts  of  racial  continuance,  and  racial  continuance  is 

assured  only  by  the  production  of  new  individuals.  In 
the  simplest  one-celled  organisms  all  individuals  are  alike,  and  reproduc- 
tion takes  place  by  a  simple  splitting  of  the   individual  body  into  two 


158       PHYSIOLOGY  ■    THE  VITAL  PROCESSES  IN  HEALTH. 


bodies.  In  many-celled  organisms  sexes  exist,  individuals  are  either  male 
or  female,  and  reproduction  is  more  complicated.  The  essence  of  sexual 
reproduction  consists  in  the  production  by  the  two  sexes  of  germ-cells 
(the  ovum,  or  egg,  by  the  female,  and  the  spermatozoon  by  the  male),  the 
union  of  these  two  germ-cells,  and  the  growth  therefrom  of  the  new  indi- 
vidual. The  reproductive  organs  subserve  these  three  functions.  For 
the  structure  and  relations  of  the  organs  the  reader  is  referred  to  the 
article  on  The  Anatomy  of  the  Human  Body. 

The  human  egg  (Fig.  47)  is  a  soft,  delicate,  spherical  cell,  about  y^-g- 
of  an  inch  in  diameter.  It  consists  of  protoplasm  and  a  small  cpiantity 
of  food  substance  scattered  through  it.  In  the  egg  of 
the  fowl,  the  egg  most  familiar  to  us,  the  vital  part 
corresponding  to  the  human  ovum  is  in  the  small  whitish  spot  upon  one 
side  of  the  yolk.  To  this  is  added,  for  the  nourishment  of  the  growing 
chick,  the  rich  food-substance  of  the  yolk  and  the  white,  while  the  shell 


Egg. 


Fig.  47. — A  human  egg  much  enlarged. 
The  greater  part  of  the  egg  consists  of  finely  granular  living  protoplasm  and  coarsely  granular 
lifeless  yolk,  the  two  being  intimately  mingled  together.  "Toward  the  upper  part  of  the  figure 
lies  the  spherical  yerminal  vesicle  or  nucleus,  within  which  is  the  small,  denser  (dark)  ger- 
minal spot  or  nucleolus.  The  egg  is  surrounded  by  a  thick,  transparent  egg-membrane,  or 
zona  peUucida,  which  is  penetrated  by  fine  pore-canals,  represented  by"radiatino-  lines. 
(HaecKel.) 


is  a  protective  covering.  The  human  chick  develops  within  the  body  of 
the  mother,  hence  a  mass  of  food  within  the  egg  and  a  shell  are  unneces- 
sary.      Eggs    are    produced   within   the   ovary  of  the  mother  (Fig,  48) 


OVULATION   AND   MENSTRUATION. 


159 


throughout  the  period   of  her  sexual  life — i.  e.,  from   puberty,   at   the 
age  of  thirteen  to  seventeen,  until  the  menopause,  at  forty  to  fifty  years 


Fig.  48. — Female  reproductive  organs  (two  thirds  the  natural  size),  as  seen  from  behind. 

u,  upper  part,  and  c,  neck  of  uterus  (in  section) ;  v,  upper  part  of  vagina ;  od,  Fallopian  tubes  (left 
one  cut  oil')  opening  into  cavity  of  abdomen  atji  ;  o,  right  ovary  ;  po,  A,  accessory  ovarian  struc- 
tures ;  £,  11,  lo,  supporting  ligaments. 


of  age.  During  this  period,  at  intervals  of  about  twenty-eight  days, 
unless  interrupted  by  pregnancy  or  disorders,  an  ovum  becomes  ripe, 
breaks  through  the  ovarian  wall,  and  is  discharged  from  the  body  through 
the  Fallopian  tabes,  the  uterus,  and  the  vagina. 

Accompanying  this  discharge  characteristic  phenomena  take  place  in 
the  wall  of  the  uterus  and  elsewhere,  constituting  menstruation.  The 
lining  membrane  of  the  uterus,  which  has  become  thick 
and  swollen  with  blood,  rapidly  degenerates,  breaks 
away  from  its  attachment,  and  gradually  passes  away  from  the  body 
through  the  vagina,  accompanied  by  a  considerable  flow  of  blood  from 
the  torn  blood-vessels  of  the  uterine  wall,  and  by  mucus.  The  menstrual 
flow  occupies  upon  an  average  about  four  days.  It  is  preceded  by  a  gen- 
eral increase  in  physiological  activity  and  excitability,  by  a  rapid  pulse 
and  a  high  temperature,  and  is  accompanied  by  more  or  less  profound 
bodily  and  mental  alterations,  lassitude,  and  general  vital  depression.  The 
significance  of  this  curious  monthly  cycle,  which  is  an  inheritance  from 
our  mammalian  ancestry,  and  which  finds  its  counterpart  in  the  "  heat " 
of  animals  and  the  actual  menstrual  flow  of  the  female  monkey,  is  a  dis- 
puted question.  As  has  been  said,  menstruation  is  accompanied  by  the 
discharge  of  an  ovum,  and  there  is  no  reason  to  doubt  that  there  is  an 
important  relation  between  the  two  phenomena,  and  probably  between 
menstruation  and  pregnancy.  According  to  one  prominent  theory,  the 
tearing  loose  of  the  lining  of  the  uterine  wall  is  for  the  purpose  of  pro- 
viding a  fresh  surface  to  which  the  ovum,  if  impregnated  in  its  passage 
outward,  can  readily  attach  itself  and  there  develop.     An  antagonistic 


160       PHYSIOLOGY:    THE  VITAL  PROCESSES   IN   HEALTH. 


Fertilization. 


theory  supposes  the  thickened  uterine  membrane  to  act  as  such  a  bed  for 
the  ovum,  and  the  shedding  of  the  membrane  to  be  a  sign  that  the  ovum 
has  not  attached  itself.  Menstruation  does  not  usually  occur  during  preg- 
nancy or  nursing. 

The  spermatozoon  (Fig.  49)  is  a  curiously  modified  cell,  consisting  of 
a  flattened,  egg-shaped  head,  a  short,  rodlike  middle  piece,  and  a  long, 
slender,  delicately  tapering  tail.      Its  length   is  from 
Per  ffa  to  -g-J-j  of  an  inch.     The  tail  is  a  locomotor  organ 

solely,  and  is  capable  of  very  active  lashing  movements,  which  are  suffi- 
ciently powerful  to  propel  the  whole  spermatozoon,  when  placed  in  liquid 
or  upon  a  moist  membrane.  This  is  essential  to  insure 
the  meeting  of  the  two  germ-cells.  The  spermatozoa  are 
produced  in  the  testis  along  with  some  liquid  ;  the  mix- 
ture, together  with  liquid  secreted  by  other  glands  con- 
nected with  the  generative  passages,  constituting  a  thick, 
whitish  fluid,  the  semen.  The  semen  is  produced  from 
puberty  until  old  age,  and  is  stored  in  the  testis,  in  its 
duct,  and  in  the  seminal  vesicles. 

During  sexual  union  the  seminal  fluid  is  transferred 
from  the  body  of  the  male  to  the  vagina  of  the  female, 
whence  the  spermatozoa  make  their  way 
by  their  own  movements  through  the 
uterus  and  along  the  Fallopian  tubes.  Here  they  may 
live  for  several  days  awaiting  the  discharge  of  an  ovum. 
If  the  egg  appears,  one  spermatozoon  bores  its  way  into 
it,  the  tail  being  left  outside  to  die.  The  head  of  the 
spermatozoon  and  the  ovum  fuse  together,  the  process 
being  called  fertilization,  and  the  development  of  the 
new  being  is  then  ready  to  begin.  Thus  it  is  seen  that 
every  individual  begins  his  life  as  a  single  minute  cell 
within  the  body  of  the  mother,  which  cell  consists  of 
material  substance  derived  partly  from  the  body  of  the 
father,  partly  from  that  of  the  mother. 

The  phenomena  of  embryonic  growth  that  follow 
fertilization  are  probably  the  most  remarkable  of  all 
vital  phenomena,  and  are  of  surpassing  interest.  The 
impregnated  ovum  divides  into  two  cells,  each  of  the  two  into  two, 
making  four  cells,  the  four  into  eight,  the  eight  into  sixteen,  and  so  on 
(Fig.  50),  this  process  being  called  segmentation.  At 
the  same  time  the  segmenting  egg  passes  along  the 
Fallopian  tube  and  enters  the  uterus,  or  womb,  which  for  the  subsequent 
nine  months  is  to  be  the  home  of  the  future  human  being.  The  egg 
attaches  itself  firmly  to  the  uterine  wall  and  becomes  embedded  in  and 


Fig.  49. — Human 

spermatozoa. 

(Magnified  600 

diameters.) 

1,  flat  view ;  % 
side  view.  A, 
head ;  B,  mid- 
dle piece;  C, 
tail ;  D.  termi- 
nal filament 
(From  Flint, 
after  Landois.) 


Growth. 


PHENOMENA  OF  EMBRYONIC   GROWTH. 


161 


covered  over  by  its  lining  membrane,  which  in  the  meantime  has  be- 
come congested  with  blood,  and  has  grown  rapidly,  as  before  menstrua- 


Placenta. 


Fig.  50. — Segmentation  of  the  egg  of  the  rabbit. 
a,  two-celled  stage :  b,  four-celled  stage ;  c,  eight-celled  stage  ;  d,  e,  further  stages ;  ect,  cell  destined 
to  give  rise  to  ectoderm,  or  outer  layer  of  embryonic  cells;  ent,  cell  destined  to  give  rise  to 
entoderm,  or  inner  cells ;  zp,  zona  pellucida ;  pgl,  polar  globules,  or  cast-on"  bits  of  nucleus. 
(Much  magnified.) 

tion.  The  number  of  cells  in  the  young  and  delicate  embryo  increases ; 
the  cells  become  differentiated  in  size,  shape,  and  function,  and  form 
the  tissues  and  the  organs.  The  embryonic  heart  begins  to  beat,  by 
degrees  the  human  form  is  evolved,  and  the  child  is  gradually  prepared 
for  its  entrance  into  the  world. 

Into  the  details  of  embryonic  growth  we  can  not  here  go.  One 
point  we  may  refer  to — that  is,  the  anatomical  and  physiological  con- 
nection that  exists  between  the  infant  and  the  mother. 
The  embryo,  surrounded  by  fluid  and  by  membranes, 
develops  within  a  cavity  in  the  wall  of  the  womb,  and  by  its  contin- 
ued growth  and  increase  in  size  the  original  cavity  of  the  womb  be- 
comes practically  obliterated  (Fig.  51).  The  embryo,  however,  is  free 
from  actual  attachment  to  the  maternal  tissues  except  at  one  point — the 
navel  of  the  child  is  connected  by  the  umbilical  cord  (Fig.  51)  to  a  con- 
siderable area  of  the  uterine  wall,  the  attachment  being  the  placenta.  The 
placenta  is  the  embryonic  organ  of  nutrition — food  reception,  respira- 
tion, and  excretion.  The  embryo,  inclosed  as  it  is,  is  incapable  of  using 
its  alimentary  canal,  lungs,  and  organs  of  excretion,  and  hence  all  its 
income  and  outgo  must  be  carried  on  between  its  own  blood  and  that 
i  >f  its  mother.  The  blood  and  the  blood-vessels  of  the  two  are  quite 
distinct  from  each  other;  the  child  has  its  own  intrinsic  blood  system. 
But  in  addition  to  this  the  large  umbilical  artery  passes  from  its  body 
along  the  umbilical  cord,  there  to  end  in  the  placenta  in  thin-walled 
13 


162      PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN  HEALTH. 

tufts  that  penetrate  into  large  spaces  filled  with  the  mother's  blood. 
The  embryonic  and  the  maternal  blood  are  separated  by  a  thin  mem- 
brane only,  that  allows  ready  diffusion  of  food,  oxygen,  carbonic  acid, 
and  wastes.     The  embryonic  blood  is  purified  and  refreshed  at  the  ex- 


Fig.  51. — Embryo  within  the  uterus."*  (Diagrammatic.) 
c,  embryo  ;  ?',  alimentary  canal  of  embryo;  r,o, m\ m,  various  structures  composing  the  umbilical 
cord;  q*  n\  q,  a,  placenta;  m,a\q\  s,  membranes  surrounding  embryo;  ft,  ft,  lining  membrane 
of  uterus :   at  each  side  extends  off  a  Fallopian  tube,  and  below  is  the  neck  of  the  uterus. 
(Liegeois.) 

pense  of  the  mother,  and  is  returned  by  the  umbilical  vein  to  the  body 
of  the  child. 

The  usual  duration  of  pregnancy  is  about  forty  weeks.  Toward  its 
close  the  presence  of  rhythmically  repeated  pains  in  the  uterus  heralds 
the  birth  of  the  child.  These  pains  of  labour  are 
accompaniments  of  wavelike  contractions  that  pass 
over  the  muscular  uterine  walls.  With  each  succeeding  contraction 
the  walls  press  closer  and  closer  upon  the  foetus,  the  opening  of  the 
womb  and  the  vagina  relax,  and  the  child  is  slowly  and  painfully 
forced  upon  the  world.  The  first  breath  is  drawn  into  the  lungs,  and 
the  infant  announces  its  arrival  by  cries  of  distress.  The  umbilical 
cord  is  tied  and  cut ;  a  few  minutes  after  birth  the  placenta  is  expelled  ; 
and  after  this  the  wounded  uterus  gradually  heals. 


PHYSIOLOGICAL  LIFE  OF  THE  INDIVIDUAL.  163 

Usually  for  some  time  after  birth  the  child  continues  to  depend  upon 
the  mother  for  its  sustenance.  During  pregnancy  the  mammary  glands 
of  the  mother,  which  form  the  breasts,  increase  in  size 
and  in  functional  power,  and  at  the  time  of  birth  they 
are  capable  of  secreting  the  milk  that  the  child  requires.  Human  milk  is 
essentially  not  unlike  the  milk  of  other  female  animals.  It  contains,  how- 
ever, somewhat  more  sugar,  and  is  more  watery  than  cow's  milk. 

We  can  not  bring  this  chapter  to  a  close  more  fittingly  than  by  a  brief 

review  of  the  physiological  life  of  the  individual  and  a  reference  to  some 

of  its  more  special  features.     A  lifetime  may  be  di- 

Individual  vided  roughly  into  three  periods :  those  of  youth,  of 
middle  life,  and  of  old  age.  Youth  is  the  period 
of  growth,  middle  life  that  of  maturity,  and  old  age  that  of  de- 
cline. A  sharp  distinction  between  these  three  is,  however,  impossible  ; 
the  coming  of  age  of  the  individual  at  twenty-one  years  is  a  convenient 
legal  fact,  but  not  a  principle  of  nature.  Growth  in  height  may  con- 
tinue until  about  twenty-five  years  of  age,  and  after  fifty  years  a  dimi- 
nution of  stature  may  follow.  Weight  may  continue  to  increase  until 
forty  years,  and  after  sixty  years  it  may  decrease.  From  about  ten  to 
fifteen  years  of  age  girls  grow  more  rapidly  than  boys,  the  year  of  most  ac- 
tive growth  in  girls  being,  in  Europe  and  the  United  States,  the  thirteenth. 
After  the  fifteenth  year  boys  surpass  girls  in  rate  of  growth,  their  most 
active  year  being  the  sixteenth.  Girls  reach  puberty  before  boys,  and 
attain  their  complete  growth  at  an  age  three  or  four  years  younger  than 
boys.  The  metabolism  during  the  period  of  youth  is  of  necessity  largely 
constructive,  during  middle  life  the  constructive  and  destructive  phases 
balance  each  other,  and  in  old  age  destructive  metabolism,  with  possible 
fatty  or  calcareous  degeneration  of  tissues,  becomes  more  prominent.  In 
comparison  with  the  young  of  most  other  animals,  the  human  infant 
comes  into  the  world  very  immature  and  helpless.  Hence  the  period  of 
youth  is  largely  devoted  to  a  perfecting  of  the  various  functions,  and  es- 
pecially to  the  forming  of  associations  and  the  laying  out  of  paths  of 
greater  and  less  resistance  within  the  central  nervous  system — in  a  word, 
to  the  formation  of  habits.  Many  functions  vary  in  their  activity  peri- 
odically. As  examples  of  such  variations  may  be  mentioned  the  common 
increase  of  weight  in  winter  and  decrease  in  summer,  the  monthly  men- 
strual flow,  a  regular  daily  variation  in  temperature,  such  that  the  highest 
temperature  occurs  between  9  a.  m.  and  6  p.  m.,  the  lowest  between 
11  p.  m.  and  3  a.  m.,  and  a  similar  daily  variation  in  the  pulse  rate 
and  in  the  rate  and  depth  of  respiration. 

Of  all  the  daily  rhythms,  the  alternation  of  sleep  and  waking  is  the 
most  striking.     Sleep  is,  in  brief,  a  profound  depression  of  the  activities 


164      PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN  HEALTH. 

of  the  central  nervous  system.  The  muscular  reflexes,  the  activities  of 
the  sense-organs,  respiration,  secretion  by  the  glands,  and  general  meta- 
bolism are  all  depressed,  perhaps  as  the  result  of  the  cen- 
tral depression  ;  but  the  absence  of  consciousness— that 
is,  the  depression  of  the  activities  of  the  psychic  cells — is  the  most  pro- 
nounced feature  of  sleep.  That  the  psychic  activity  is  not  always  wholly 
eliminated  is  evidenced  by  the  occurrence  of  dreams,  but  the  fragmentary 
and  grotesque  character  of  dreams  indicates  that  but  a  few  brain-cells 
are  engaged  in  their  production.  The  reason  for  this  periodic  depression 
of  brain  activity  has  been  long  sought,  but  with  little  success.  Perhaps 
the  accumulation  within  the  body  of  the  waste  products  of  protoplasmic 
activity  causes  a  temporary  paralysis  of  the  brain-cells,  lasting  until  the 
wastes  are  removed.  However  this  may  be,  the  amount  of  blood  in  the 
brain  is  probably  considerably  less  during  the  sleeping  than  during  the 
waking  hours,  and  this  may  of  itself  cause  cerebral  depression. 

Sooner  or  later,  even  if  disease  and  accident  are  safely  overcome,  the 
vital  machine  is  doomed  to  wear  out,  and  death  follows.  Physiologists 
recognise  two  kinds  of  death  :  death  of  the  individual 
and  death  of  the  tissues.  The  individual  dies  when  the 
heart  ceases  to  beat  and  to  supply  the  whole  body  with  food  and  oxygen. 
But  the  individual  tissues  vary  very  greatly  in  the  time  of  their  death. 
The  nervous  tissues  die  almost  immediately,  but  the  muscles  are  capable 
of  contraction  upon  artificial  stimulation  for  a  considerable  time  after  the 
thinking  man  has  forever  ceased  his  activities.  Probably  not  for  several 
hours  do  all  the  chemical  changes  take  place  within  the  protoplasm  that 
are  the  evidence  of  the  passing  away  of  that  mysterious,  unexplained 
condition  that  we  are  wont  to  call  life. 


CHAPTEK  VI. 
HEREDITY. 

In  the  vast  world  of  life  nothing  seems  more  marvellous  than  the  fact 
of  inheritance.     Like  produces  like  ;  a  child  resembles  its  parents. 

This  fact  is  easily  comprehensible  in  the  one-celled  organisms,  where 
the  parent  cell,  after  a  period  of  activity,  puts  an  end  to  its  individuality 
by  an  equal  division  of  its  whole  body  into  two  similar 
Biological  Fact      offspring,  and  nothing  is  lost.     But  in  the  higher  and 
larger  animals,  composed  of  countless  millions  of  cells 
that  are  specialized  in  a  great  variety  of  ways,  the  vital  material  con- 
tributed by  the  parents  consists  solely  of  the  two  microscopic  germ  cells, 


INHERITANCE  A  BIOLOGICAL  FACT.  165 

and    from  this    infinitesimal    beginning    there    comes  an  organism  that 

resembles,  often    in    minute  and    unimportant    details,  the    parents,  the 

grandparents,  and  even  remote  ancestors,  and  that  does  not  resemble 

other  races.     Here,  if  anywhere  in  our  search  after  natural  causes  of 

things,  it  seems  necessary  to  bring  into  causal  relation  the  supernatural, 

and  to  hide  our  helplessness  in  an  assumption  of  some  mysterious  guiding 

force  that  is  distinct  from  vital  matter  and  that  we  know  not  of.     Truly, 

we  know  little  of  natural  law,  but  the  restless  scientific  spirit  of  man  is 

ill  content  with  a  supernatural  explanation  of  heredity.     Inheritance  is  a 

biological  fact,  and  must  be  explained  in  accordance  with  biological  laws. 

Before  proceeding,  however,  to  the  explanation,  let  us  review  in  some 

detail  the  facts  that  must  be  explained.     We  have  said  that  like  produces 

like.     This   statement   must  be  accepted   with  certain 

acsoj  modifications.     Obviously,  no  two  individuals  are  like 

Inheritance.  w  _  .         . 

each  other  in  all  points  ;  twins,  indeed,  may  be  "as  like 

as  two  peas,"  and  yet  one  is  not  the  mirrored  image  of  the  other.  As  a 
general  law,  a  child  resembles  its  parents  more  closely  than  it  resembles 
any  other  individual.  Between  its  parents,  the  resemblance  is  in  some 
cases  more  strongly  in  favour  of  the  father,  in  other  cases  in  favour  of 
the  mother ;  in  still  others,  both  paternal  and  maternal  characteristics 
seem  to  be  present  in  approximately  equal  proportions ;  rarely  is  the 
child  "  the  exact  image  "  of  either  parent.  At  times,  however,  the  off- 
spring seems  to  possess  almost  or  quite  none  of  the  parental  qualities,  but 
to  show  likeness  to  grandparents  or  great-grandparents,  either  as  regards 
general  or  as  regards  particular  features  or  qualities.  The  reappearance 
of  an  ancestral  quality  thus,  after  having  lapsed  in  one  or  more  genera- 
tions, is  called  atavism  or  reversion,  and  the  subject  forms  one  of  the  in- 
teresting chapters  in  the  story  of  heredity.  Lastly,  in  rare  cases  a  child 
seems  to  be  a  veritable  "  black  sheep,"  and  to  possess  no  qualities  what- 
ever that  ally  it  to  its  progenitors.  In  speaking  of  inherited  resemblances, 
we  do  not  mean  to  confine  ourselves  to  mere  anatomical  matters ;  likeness 
of  feature,  of  form,  of  size,  of  structure,  is  most  obvious,  and  is  the  kind 
of  resemblance  most  commonly  sought  for.  But  likeness  in  things 
physiological  and  things  psychological,  in  the  mode  of  working  of  the 
body  and  of  the  mind,  in  things  moral,  in  peculiarities  of  temperament, 
is  as  common,  if  not  so  readily  perceived.  In  fact  it  is  difficult,  if  not 
quite  impossible,  to  draw  the  line  between  features  and  qualities  of  the 
parent  that  are  heritable  and  those  that  are  not  so. 

The  question  of  the  inheritance  of  disease  has  been  much  debated.  It 
seems  to  be  a  fact  that  the  germs  of  certain  infectious  diseases,  such  as 
syphilis,  may  be  conveyed  to  the  child's  organism  within  the  parent's  germ 
cells,  or  even  directly  from  the  mother's  body  to  the  growing  embryo. 
It  seems  also  probable  that  predisposition  to  certain  diseases,  in  the  form 


166       PHYSIOLOGY  :    THE  VITAL  PKOCESSES  IN  HEALTH. 

of  constitutional  weakness  and  diminished  power  of  resistance  to  the  dis- 
ease germs,  is  heritable.      Probably  the  apparent  transmission  of  con- 
sumption is  thus  to  be  explained.     Inherited  predispo- 

n  en  a    e  oj       gition  and  the  fact  that  the  child  is  usually  brought  up 

Diseases.  ,  j  o         i 

in  the  home  of   its    consumptive    parents,  with  every 

opportunity  of  acquiring  the  very  germ  that  it  can  not  successfully  resist, 
are  probably  responsible  for  many  attacks  of  this  fatal  malady.  Predis- 
position to  nervous  diseases,  to  insanity,  to  suicide,  and  to  crime,  are 
certainly  transmissible. 

A  second  much-debated  question  is  that  as  to  whether  new  character- 
istics acquired  by  a  parent  may  be  transmitted  to  his  child.     To  the  mind 

of  the  layman,  and  at  first  thought  the  question  seems 
Inheritance  of  Ac-  j  j.  , -,      ,  1     '  ui      ■ 

.    , ,,,     '  superfluous,  for  apparently  there  are  innumerable  m- 

quired  Characters.  r  '  .     .  ™ 

stances  of  such  transmission  about  us — the  affirmative 

side  of  the  question  goes  without  saying.  But  when  we  examine  the  evi- 
dence carefully  we  are  surprised  at  its  inconclusiveness.  In  one  case  the 
peculiarity  is  found,  after  all,  to  be  not  one  acquired  by  the  parent,  but  one 
possessed  also  by  other  ancestors — to  be  inherent  in  the  race  ;  in  another 
case,  what  is  inherited  may  easily  be  a  tendency,  a  strength  or  weakness 
of  mind  or  character,  that  with  a  suitable  environment  may  show  in  the 
child  in  the  same  striking  manner  as  in  the  parent,  and  yet  that  is  in  no 
sense  an  inheritance  of  anything  acquired  by  the  parent — such,  for  ex- 
ample, as  a  taste  for  strong  drink  ;  in  another  case,  the  parent's  peculiarity 
may  prove,  upon  examination,  to  have  been  obtained  subsequent  to  the 
birth  of  the  child  !  And  thus,  one  by  one,  innumerable  cases  of  seeming 
inheritance  of  acquired  characters  have  been  disproved.  Experimental 
investigation  has  proved  equally  inconclusive.  For  centuries  the  Chinese 
have  compressed  and  distorted  the  feet  of  their  girls ;  yet  the  feet  of 
Chinese  girls  born  at  the  present  time  are  apparently  not  different  from 
those  of  a  generation  a  thousand  years  ago.  Numerous  researches  made 
in  recent  years  upon  the  dehorning  of  cattle  and  the  removal  of  the  tails 
of  mice  for  a  long  series  of  generations  have  resulted  in  no  reduction  of 
the  horns  in  the  one  case  or  of  the  tails  in  the  other.  Hence,  though 
there  are  still  many  seeming  arguments  in  favour  of  use  and  disuse  as 
important  factors  in  evolution  and  in  heredity,  there  are  many  persons 
that  exclude  these  utterly  from  their  creed,  and  so  the  matter  of  the 
inheritance  of  acquired  characters  must  be  left  for  the  present  undecided. 
In  view  of  the  above  facts  it  seems  idle,  and  it  is  certainly  misleading, 
to  talk  of  the  equality  of  men,  for  Nature  has  established  classes  that  are 
much  more  firmly  ordained,  than  are  those  of  society. 
i  y  pouerju  ^^  environment  of  the  child  and  the  man  has  much  to 
do  with  what  the  individual  accomplishes  in  the  world,  but  the  environ- 
ment works  upon  material  that  already  is  moulded  in  great  part  by  past 


THEORIES  OF  HEREDITY.  167 

generations.  The  inherited  qualities  of  one  person  place  him  at  once 
well  in  advance  in  the  struggle  for  existence  ;  those  of  another  too  often 
prove  a  serious  handicap.  Heredity  is  a  powerful  factor  in  human 
progress.  It  is  to  be  hoped  that  a  greater  knowledge  of  its  facts  and 
principles  will  gradually  modify  existing  educational  systems  for  the 
young  and  penal  systems  for  criminals. 

How,  now,  may  the  above  facts  of  inheritance  be  explained  ?  Leav- 
ing aside  all  metaphysical  and  theological  theories  as  unbiological  and 
inadequate,  we  must  start  with  the  belief,  incredible  as 
it  may  seem,  that  the  bits  of  protoplasm  that  constitute 
the  ovum  and  the  spermatozoon  contain  within  themselves  in  some  form 
the  qualities  of  the  mother  and  the  father  respectively,  and  to  some  ex- 
tent of  more  distant  progenitors.  Probably  we  can  go  even  further  than 
this,  for  there  is  much  evidence  that  the  real  carrier  of  the  hereditary 
qualities,  the  germ-plasm,  is  confined  to  the  nuclei  alone  of  the  two  germ 
cells.  We  have  already  seen  that  the  tail  of  the  spermatozoon  is  a  loco- 
motor organ,  and  dies  after  conveying  the  head  to  the  ovum  ;  the  head 
consists  chiefly  of  nucleus,  and  it  alone  enters  the  egg.  Likewise,  within 
the  ovum  the  mass  of  egg  substance  seems  to  subserve  nutritive  and  other 
purposes,  while  the  nucleus  alone  is  probably  essentially  hereditary.  The 
fusion  of  the  spermatozoon  and  the  ovum  is  a  fusion  of  their  nuclei,  and 
the  segmentation  of  the  egg  is  primarily  a  nuclear  phenomenon. 

But  how  is  it  possible  for  the  minute  germ-plasm  to  obtain  and  to  hold 
the  parental  qualities  ?     This  problem  has  been  the  subject  of  much  spec- 
ulation, especially  since  the  writings  of  Darwin  set  men 
Darwin's  Theory  , ,  .    •,  .  <■  , ,'  ■    •        r  •  j  .  i  1       • 

„  „  .to  thinking  of  the  origin  of  species  and  the  mechanism 

of  Pangenesis.  °  . 

of  descent.  In  his  book  entitled  The  Variation  of 
Animals  and  Plants  under  Domestication,  published  in  1868,  Darwin 
gave  to  the  world  his  own  hypothesis  of  inheritance  in  a  chapter  entitled 
Provisional  Hypothesis  of  Pangenesis.  In  brief,  this  theory  supposes 
that  all  parts  of  the  body  are  giving  off  at  all  times  excessively  minute 
particles,  the  germs  of  the  various  cells,  or  gemmules,  as  Darwin  called 
them  ;  these  pass  into  the  circulating  blood,  are  carried  to  the  repro- 
ductive organs,  and  become  a  part  of  the  germ  cells.  Hence  every  germ 
cell  is  a  mass  of  germs  of  the  body  cells  of  the  parent,  together  with 
some  gemmules  of  more  remote  ancestors,  and  hence  the  fertilized  ovum 
contains  the  potentialities  of  the  image  of  either  parent  and  of  some  more 
ancient  ancestral  characteristics.  Darwin's  gemmules  are  somewhat  of 
the  nature  of  Herbert  Spencer's  hypothetical  "  physiological  units,"  which, 
before  Darwin  wrote,  were  used  by  Spencer  to  explain  heredity.  Dar- 
win's hypothesis  allows  the  transmission  of  characteristics  that  are  ac- 
quired by  the  parent ;  for  an  alteration  of  any  part  by  use,  disuse,  loss,  or 
injury  might  cause  altered  gemmules  to  go  to  the  germ  cells,  and  the 


168       PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN  HEALTH. 

child  might  then  show  in  the  corresponding  part  of  his  body  the  effects 
of  use,  disuse,  loss,  or  injury  in  the  parent.  Darwin's  theory  is  purely 
speculative.  Neither  he  nor  later  biologists  have  known,  as  an  observed 
fact,  any  such  giving  off  of  germinal  units  as  he  assumes.  The  theory 
has  not  been  generally  accepted  ;  for,  apart  from  the  inconceivability  of 
the  presence  of  so  vast  a  number  of  germs  within  a  single  minute  cell, 
many  facts  render  it  improbable.  But  it  has  served  a  useful  purpose,  as 
doubtless  Darwin  intended  it  should  •  serve,  in  stimulating  thought  and 
investigation. 

Since  the  appearance  of  Darwin's  provisional  hypothesis  a  large  lit- 
erature upon  heredity  has  appeared,  and  several  theories  have  been  pro- 
posed.    Some  of  these  follow  Darwin  and  attempt  to 
Weismann's  Theory.  ,,,.■..  n  .  ,n  j.  ■-, 

supply  lacks  in  Ins  explanation  ;  others  are  tar  removed 

from  his  speculations.  We  have  here  space  to  mention  but  one  of  these, 
which  has  attracted  wider  attention  than  any  other,  and  which  has  been 
the  subject  of  widespread  and  earnest  discussion.  This  is  the  theory  of 
Prof.  Weismann,  of  the  German  University  of  Freiburg,  who  since  1881 
has  published  several  volumes  of  essays  upon  this  and  kindred  subjects. 
Weismann  is  a  careful  and  conscientious  thinker.  His  theory  has  been  de- 
veloped gradually,  and,  as  the  significance  of  his  main  conceptions  has 
broadened  with  continued  thought  and  investigation,  his  later  writings 
contain  modifications  of  his  earlier  views. 

The  nucleus  of  Weismann's  theor}*  is  expressed  by  his  own  phrase, 
"  the  continuity  of  the  germ-plasm."     This  phrase  means  that,  according 

to  his  idea,  the  germinal  or  hereditary  substance  that  is 
The  Continuity  of  ,    .       ,,  ,,  ,         ..        »  .     ,. 

„         7  present  -in  the  ovum  or  the  spermatozoon  of  any  indi- 

vidual, whether  man,  lower  animal,  or  plant,  has  come 
directly  from  the  germinal  substance  of  the  parent  of  the  individual ;  that 
the  germinal  substance  of  the  parent  likewise  has  come  directly  from 
that  of  the  parent's  parent ;  that  the  germinal  substance  of  any  individual 
may  be  traced  through  parent,  grandparent,  great-grandparent,  and  so 
backward  to  the  remotest  ancestors  ;  that,  in  a  word,  germinal  substance, 
arising  far  back  in  the  lowly  and  primitive  predecessors  of  existing 
organisms,  has  been  and  is  continuous  in  each  line  of  descent  throughout 
all  generations.  Hence,  germinal  substance  does  not  arise  de  novo ;  it 
does  not  originate  within  a  body  by  the  concourse  of  multitudinous 
minute  "gemmules"  that  are  given  off  from  all  parts  of  the  body,  but  it 
is  derived  solely  from  pre-existing  germinal  substance ;  in  this  sense  all 
individuals  are  powerfully  and  equally  "  blue-blooded."  Every  individual, 
therefore,  begins  life  as  a  minute  mass  of  germ-plasm,  that  consists  partly 
of  male  germ-plasm  and  partly  of  female  germ-plasm,  and  that  in  the 
case  of  the  human  being  is  destined  to  grow  and  develop  into  the  child 
within  the  mother's  womb.     As  growth  and  development  go  on,  much  of 


WEISMANN'S  THEORY.  169 

the  germ-plasm  differentiates,  and  produces  the  various -cells,  tissues,  and 
organs  of  the  child's  body.  A  small  portion,  however,  remains  undiffer- 
entiated, and  takes  up  its  residence  in  the  essential  reproductive  organ  of 
the  child — in  the  ovary,  if  the  child  be  a  girl,  in  the  testis  if  a  boy  ;  and 
this  undifferentiated  residue  which,  as  has  been  seen,  has  come  directly 
from  the  parents,  is  the  germ  of  the  future  progeny  of  the  child. 

Hence,  according  to  Weismann's  theory,  the  body  of  every  individual 
consists  of  two  kinds  of  protoplasm,  viz.,  germ-plasm,  or  hereditary  sub- 
stance, which  in  its  undifferentiated  form  resides  within 
Germ-plasm  and       . ,  , .   ,  ,  ,.,,,.. 

Somatoplasm  essential  reproductive  organ,  and  is  the  derivative 

of  past  and  the  progenitor  of  future  races ;  and  body- 
plasm,  or  somatoplasm,  which  is  the  protoplasm  of  the  rest  of  the  body, 
the  muscles,  the  glands,  the  heart,  and  the  brain.  The  latter  serves  the 
daily  needs  of  the  individual  and  dies  when  the  individual  dies.  The 
former  subserves  reproduction  alone ;  if  the  individual  reproduces  its 
kind,  some  of  the  germ-plasm  is  passed  on  to  the  descendant ;  if  the  indi- 
vidual does  not  reproduce,  the  germ-plasm  dies  with  him,  and  that  par- 
ticular line  of  descent  is  forever  ended. 

Thus  far  Weismann's  theory  seems  readily  comprehensible  and  reason- 
able.   But  let  us  carry  it,  as  its  author  does,  a  little  farther.    What  relation 

within  an  individual  body  do  these  two  kinds  of  proto- 

e  a  wn  of         plasm — the  germ-plasm  and  the  somatoplasm — bear  to 
Germ-plasm  and       *■  °   _^    J  .  r 

Somatoplasm.        eacn  other  {     Do  changes  in  the  one  necessarily  affect 

the  other  ?  Weismann  believes  that  the  two  are  quite 
independent  of  and  distinct  from  each  other.  They  are  nourished  by  the 
same  nutrient  blood  and  lymph,  it  is  true,  and  any  general  alteration  of 
the  nutrient  fluids  alters  the  nutrition  of  the  two  alike ;  but  this  is  of 
comparative  unimportance.  The  important  consideration  is  that  any 
alteration  of  a  particular  part  of  the  body-protoplasm  does  not  affect  the 
germ-plasm  ;  in  the  origin  of  the  latter  from  the  parent's  germ-plasm 
instead  of  from  the  individual's  own  somatoplasm,  and  in  the  absence  of 
gemmules  passing  constantly  from  somatoplasm  to  germ-plasm,  the  latter 
can  not  reflect  the  condition  of  the  former.  Hence,  the  loss  of  a  limb  by 
accident,  the  gain  of  strength  in  particular  muscles  by  athletic  exercise, 
the  acquisition  of  an  art,  like  piano-playing,  which  consists  of  delicate 
and  intricate  muscular  and  nervous  adjustments,  the  long-continued 
mental  development  of  the  man  or  the  woman  which  carries  with  it 
molecular  adjustments  of  brain  substance,  the  practice  of  a  trade  or  pro- 
fession which  develops  abnormally  certain  organs,  or  tissues,  or  cells,  and 
allows  other  organs,  tissues,  or  cells  to  degenerate — all  these  affect  the 
germ-plasm  in  no  wise,  and  can  not  be  transmitted  to  the  descendants  of 
the  individual.  Characteristics  acquired  by  the  parent  are,  therefore,  not 
inherited  by  the  child.     The  non-inheritance  of  acquired  characters  is 


170       PHYSIOLOGY  :    THE  VITAL  PROCESSES  IN  HEALTH. 

the  second  fundamental  postulate  of  Weismann's  theory,  and  is  to  be 
placed  beside  that  of  the  continuity  of  germ-plasm. 

Why,  then,  are  not  all  children  alike  mentally,  morally,  and  physi- 
cally ?     From  a  common  origin  in  the  early  history  of  organic  beings, 

with  a  descent  through  numberless  generations  by  a 
Variation 

continuity  of  germ-plasm  that  leads  a  charmed  life  of  a 

certain  degree  of  independence  of  environmental  changes,  how  is  varia- 
tion possible,  and  how  may  the  differences  of  individuals  and  of  races  be 
accounted  for  ?  In  the  first  place,  germ-plasm  is  not  wholly  independent 
of  environmental  changes.  It  is  capable  of,  and  is  constantly  undergoing, 
alteration  as  the  result  of  general  nutritional  alterations  taking  place  in 
the  body  in  which  it  is  housed.  The  germ-plasm  of  one  line  of  descent 
must  therefore  necessarily  differ  somewhat  from  that  of  another  line  of 
descent,  since  the  environment  of  the  one  differs  from  that  of  the  other. 
In  the  second  place — and  here  we  have  the  chief  source  of  variation — the 
individual  is  the  result  of  a  fusion  of  two  varieties  of  germ-plasm,  that  of 
the  father  and  that  of  the  mother,  each  with  a  history  and  capabilities 
differing  from  those  of  the  other.  Therefore  the  resultant  organism, 
while  possessing  ancestral  qualities,  must  possess  them  in  a  combination 
that  has  never  before  existed  ;  it  must  differ  from  any  organism  that  has 
preceded  it ;  and  hence  it  follows  that  no  two  individuals,  or  two  species, 
or  two  races  can  be  alike. 

These,  then,  are  the  elements  of  Weismann's  theory :  continuity  of 
germ-plasm  and  non-inheritance  of  acquired  characters  tending  to  pre- 
serve the  uniformity  of  the  race  ;  slight  germinal  changes  and  sexual  re- 
production tending  to  destroy  that  uniformity.*  During  the  past  ten  years 
and  more  the  theory  in  its  manifold  details  has  been  fought  over  with 
great  vigour  by  its  friends  and  its  foes.  The  contest  has  been  especially 
warm  over  the  question  of  the  inheritance  of  acquired  characters. 

We  have  thus  touched  very  incompletely  upon  some  of  the  principles 
and  problems  and  attempted  explanations  of  the  mystery  of  inheritance. 
To  the  scientist  a  mystery  is  always  inviting,  and  to  the  modern  biologist 
that  of  heredity  is  especially  so.  What  is  needed  most  is  careful,  truth- 
ful observation  and  recording  of  the  facts  that  are  all  about  us,  and  well- 
directed,  painstaking  and  unprejudiced  physiological  experimentation  as 
to  hereditary  possibilities.  Darwin,  Galton,  Weismann,  and  others  have 
done  much  of  the  former  service ;  the  latter  belongs  largely  to  the  science 
of  the  future. 

*  Essays  upon  Heredity  and  Kindred  Biological  Problems.  By  August  Weismann. 
Authorized  translation.    Vol.  I,  1889 ;  Vol.  II,  1892.    Oxford,  The  Clarendon  Press. 

The  Germ-plasm  :  A  Theory  of  Heredity.  By  August  Weismann.  Authorized  trans- 
lation.   1893.    New  York,  Charles  Scribner's  Sons. 


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